PUFA polyketide synthase systems and uses thereof

ABSTRACT

The invention generally relates to polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems isolated from or derived from non-bacterial organisms, to homologues thereof, to isolated nucleic acid molecules and recombinant nucleic acid molecules encoding biologically active domains of such a PUFA PKS system, to genetically modified organisms comprising PUFA PKS systems, to methods of making and using such systems for the production of bioactive molecules of interest, and to novel methods for identifying new bacterial and non-bacterial microorganisms having such a PUFA PKS system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/124,800, filed Apr. 16, 2002, now U.S. Pat. No. 7,247,461, entitled “PUFA Polyketide Synthase Systems and Uses Thereof,” which claims the benefit of priority under 35 U.S.C. § 119(e) to: U.S. Provisional Application Ser. No. 60/284,066, filed Apr. 16, 2001, entitled “A Polyketide Synthase System and Uses Thereof”; U.S. Provisional Application Ser. No. 60/298,796, filed Jun. 15, 2001, entitled “A Polyketide Synthase System and Uses Thereof”; and U.S. Provisional Application Ser. No. 60/323,269, filed Sep. 18, 2001, entitled “Thraustochytrium PUFA PKS System and Uses Thereof”. U.S. application Ser. No. 10/124,800, is also a continuation-in-part of U.S. application Ser. No. 09/231,899, now U.S. Pat. No. 6,566,583, filed Jan. 14, 1999, entitled “Schizochytrium PKS Genes”. Each of the above-identified patent applications is incorporated herein by reference in its entirety.

This application does not claim the benefit of priority from U.S. application Ser. No. 09/090,793, filed Jun. 4, 1998, now U.S. Pat. No. 6,140,486, although U.S. application Ser. No. 09/090,793 is incorporated herein by reference in its entirety.

REFERENCE TO SEQUENCE LISTING

This application contains a Sequence Listing submitted as an electronic text file named “2997-29_corrected_ST25.txt”, having a size in bytes of 280 kb, and created on 4 Mar. 2007. The information contained in this electronic file is hereby incorporated by reference in its entirety pursuant to 37 CFR §1.52(e)(5).

FIELD OF THE INVENTION

This invention relates to polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems from microorganisms, including eukaryotic organisms, such as Thraustochytrid microorganisms. More particularly, this invention relates to nucleic acids encoding non-bacterial PUFA PKS systems, to non-bacterial PUFA PKS systems, to genetically modified organisms comprising non-bacterial PUFA PKS systems, and to methods of making and using the non-bacterial PUFA PKS systems disclosed herein. This invention also relates to a method to identify bacterial and non-bacterial microorganisms comprising PUFA PKS systems.

BACKGROUND OF THE INVENTION

Polyketide synthase (PKS) systems are generally known in the art as enzyme complexes derived from fatty acid synthase (FAS) systems, but which are often highly modified to produce specialized products that typically show little resemblance to fatty acids. Researchers have attempted to exploit polyketide synthase (PKS) systems that have been described in the literature as falling into one of three basic types, typically referred to as: Type II, Type I and modular. The Type II system is characterized by separable proteins, each of which carries out a distinct enzymatic reaction. The enzymes work in concert to produce the end product and each individual enzyme of the system typically participates several times in the production of the end product. This type of system operates in a manner analogous to the fatty acid synthase (FAS) systems found in plants and bacteria. Type I PKS systems are similar to the Type II system in that the enzymes are used in an iterative fashion to produce the end product. The Type I differs from Type II in that enzymatic activities, instead of being associated with separable proteins, occur as domains of larger proteins. This system is analogous to the Type I FAS systems found in animals and fungi.

In contrast to the Type I and II systems, in modular PKS systems, each enzyme domain is used only once in the production of the end product. The domains are found in very large proteins and the product of each reaction is passed on to another domain in the PKS protein. Additionally, in all of the PKS systems described above, if a carbon-carbon double bond is incorporated into the end product, it is always in the trans configuration.

In the Type I and Type II PKS systems described above, the same set of reactions is carried out in each cycle until the end product is obtained. There is no allowance for the introduction of unique reactions during the biosynthetic procedure. The modular PKS systems require huge proteins that do not utilize the economy of iterative reactions (i.e., a distinct domain is required for each reaction). Additionally, as stated above, carbon-carbon double bonds are introduced in the trans configuration in all of the previously described PKS systems.

Polyunsaturated fatty acids (PUFAs) are critical components of membrane lipids in most eukaryotes (Lauritzen et al., Prog. Lipid Res. 40 1 (2001); McConn et al., Plant J. 15, 521 (1998)) and are precursors of certain hormones and signaling molecules (Heller et al., Drugs 55, 487 (1998); Creelman et al., Annu. Rev. Plant Physiol. Plant Mol. Biol. 48, 355 (1997)). Known pathways of PUFA synthesis involve the processing of saturated 16:0 or 18:0 fatty acids (the abbreviation X:Y indicates an acyl group containing X carbon atoms and Y cis double bonds; double-bond positions of PUFAs are indicated relative to the methyl carbon of the fatty acid chain (ω3 or ω6) with systematic methylene interruption of the double bonds) derived from fatty acid synthase (FAS) by elongation and aerobic desaturation reactions (Sprecher, Curr. Opin. Clin. Nutr. Metab. Care 2, 135 (1999); Parker-Barnes et al., Proc. Natl. Acad. Sci. USA 97, 8284 (2000); Shanklin et al., Annu. Rev. Plant Physiol. Plant Nol. Biol. 49, 611 (1998)). Starting from acetyl-CoA, the synthesis of DHA requires approximately 30 distinct enzyme activities and nearly 70 reactions including the four repetitive steps of the fatty acid synthesis cycle. Polyketide synthases (PKSs) carry out some of the same reactions as FAS (Hopwood et al., Annu. Rev. Genet. 24, 37 (1990); Bentley et al., Annu. Rev. Microbiol. 53, 411 (1999)) and use the same small protein (or domain), acyl carrier protein (ACP), as a covalent attachment site for the growing carbon chain. However, in these enzyme systems, the complete cycle of reduction, dehydration and reduction seen in FAS is often abbreviated so that a highly derivatized carbon chain is produced, typically containing many keto- and hydroxy-groups as well as carbon-carbon double bonds in the trans configuration. The linear products of PKSs are often cyclized to form complex biochemicals that include antibiotics and many other secondary products (Hopwood et al., (1990) supra; Bentley et al., (1999), supra; Keating et al., Curr. Opin. Chem. Biol. 3, 598 (1999)).

Very long chain PUFAs such as docosahexaenoic acid (DHA; 22:6ω3) and eicosapentaenoic acid (EPA; 20:5ω3) have been reported from several species of marine bacteria, including Shewanella sp (Nichols et al., Curr. Op. Biotechnol. 10, 240 (1999); Yazawa, Lipids 31, S (1996); DeLong et al., Appl. Environ. Microbiol. 51, 730 (1986)). Analysis of a genomic fragment (cloned as plasmid pEPA) from Shewanella sp. strain SCRC2738 led to the identification of five open reading frames (Orfs), totaling 20 Kb, that are necessary and sufficient for EPA production in E. coli (Yazawa, (1996), supra). Several of the predicted protein domains were homologues of FAS enzymes, while other regions showed no homology to proteins of known function. On the basis of these observations and biochemical studies, it was suggested that PUFA synthesis in Shewanella involved the elongation of 16- or 18-carbon fatty acids produced by FAS and the insertion of double bonds by undefined aerobic desaturases (Watanabe et al., J. Biochem. 122, 467 (1997)). The recognition that this hypothesis was incorrect began with a reexamination of the protein sequences encoded by the five Shewanella Orfs. At least 11 regions within the five Orfs were identifiable as putative enzyme domains (See Metz et al., Science 293:290-293 (2001)). When compared with sequences in the gene databases, seven of these were more strongly related to PKS proteins than to FAS proteins. Included in this group were domains putatively encoding malonyl-CoA:ACP acyltransferase (MAT), 3-ketoacyl-ACP synthase (KS), 3-ketoacyl-ACP reductase (KR), acyltransferase (AT), phosphopantetheine transferase, chain length (or chain initiation) factor (CLF) and a highly unusual cluster of six ACP domains (i.e., the presence of more than two clustered ACP domains has not previously been reported in PKS or FAS sequences). However, three regions were more highly homologous to bacterial FAS proteins. One of these was similar to the newly-described Triclosan-resistant enoyl reductase (ER) from Streptococcus pneumoniae (Heath et al., Nature 406, 145 (2000)); comparison of ORF8 peptide with the S. pneumoniae enoyl reductase using the LALIGN program (matrix, BLOSUM50; gap opening penalty, −10; elongation penalty −1) indicated 49% similarity over a 386 aa overlap). Two regions were homologues of the E. coli FAS protein encoded by fabA, which catalyzes the synthesis of trans-2-decenoyl-ACP and the reversible isomerization of this product to cis-3-decenoyl-ACP (Heath et al., J. Biol. Chem., 271, 27795 (1996)). On this basis, it seemed likely that at least some of the double bonds in EPA from Shewanella are introduced by a dehydrase-isomerase mechanism catalyzed by the FabA-like domains in Orf7.

Anaerobically-grown E. coli cells harboring the pEPA plasmid accumulated EPA to the same levels as aerobic cultures (Metz et al., 2001, supra), indicating that an oxygen-dependent desaturase is not involved in EPA synthesis. When pEPA was introduced into a fabB⁻ mutant of E. coli, which is unable to synthesize monounsaturated fatty acids and requires unsaturated fatty acids for growth, the resulting cells lost their fatty acid auxotrophy. They also accumulated much higher levels of EPA than other pEPA-containing strains, suggesting that EPA competes with endogenously produced monounsaturated fatty acids for transfer to glycerolipids. When pEPA-containing E. coli cells were grown in the presence of [¹³C]-acetate, the data from ¹³C-NMR analysis of purified EPA from the cells confirmed the identity of EPA and provided evidence that this fatty acid was synthesized from acetyl-CoA and malonyl-CoA (See Metz et al., 2001, supra). A cell-free homogenate from pEPA-containing fabB⁻ cells synthesized both EPA and saturated fatty acids from [¹⁴C]-malonyl-CoA. When the homogenate was separated into a 200,000×g high-speed pellet and a membrane-free supernatant fraction, saturated fatty acid synthesis was confined to the supernatant, consistent with the soluble nature of the Type II FAS enzymes (Magnuson et al., Microbiol. Rev. 57, 522 (1993)). Synthesis of EPA was found only in the high-speed pellet fraction, indicating that EPA synthesis can occur without reliance on enzymes of the E. coli FAS or on soluble intermediates (such as 16:0-ACP) from the cytoplasmic fraction. Since the proteins encoded by the Shewanella EPA genes are not particularly hydrophobic, restriction of EPA synthesis activity to this fraction may reflect a requirement for a membrane-associated acyl acceptor molecule. Additionally, in contrast to the E. coli FAS, EPA synthesis is specifically NADPH-dependent and does not require NADH. All these results are consistent with the pEPA genes encoding a multifunctional PKS that acts independently of FAS, elongase, and desaturase activities to synthesize EPA directly. It is likely that the PKS pathway for PUFA synthesis that has been identified in Shewanella is widespread in marine bacteria. Genes with high homology to the Shewanella gene cluster have been identified in Photobacterium profundum (Allen et al., Appli. Environ. Microbiol. 65:1710 (1999)) and in Moritella marina (Vibrio marinus) (Tanaka et al., Biotechnol. Lett. 21:939 (1999)).

The biochemical and molecular-genetic analyses performed with Shewanella provide compelling evidence for polyketide synthases that are capable of synthesizing PUFAs from malonyl-CoA. A complete scheme for synthesis of EPA by the Shewanella PKS has been proposed. The identification of protein domains homologous to the E. coli FabA protein, and the observation that bacterial EPA synthesis occurs anaerobically, provide evidence for one mechanism wherein the insertion of cis double bonds occurs through the action of a bifunctional dehydratase/2-trans, 3-cis isomerase (DH/2,3I). In E. coli, condensation of the 3-cis acyl intermediate with malonyl-ACP requires a particular ketoacyl-ACP synthase and this may provide a rationale for the presence of two KS in the Shewanella gene cluster (in Orf 5 and Orf 7). However, the PKS cycle extends the chain in two-carbon increments while the double bonds in the EPA product occur at every third carbon. This disjunction can be solved if the double bonds at C-14 and C-8 of EPA are generated by 2-trans, 2-cis isomerization (DH/2,2I) followed by incorporation of the cis double bond into the elongating fatty acid chain. The enzymatic conversion of a trans double bond to the cis configuration without bond migration is known to occur, for example, in the synthesis of 11-cis-retinal in the retinoid cycle (Jang et al., J. Biol. Chem. 275, 28128 (2000)). Although such an enzyme function has not yet been identified in the Shewanella PKS, it may reside in one of the unassigned protein domains.

The PKS pathways for PUFA synthesis in Shewanella and another marine bacteria, Vibrio marinus, are described in detail in U.S. Pat. No. 6,140,486 (issued from U.S. application Ser. No. 09/090,793, filed Jun. 4, 1998, entitled “Production of Polyunsaturated Fatty Acids by Expression of Polyketide-like Synthesis Genes in Plants”, which is incorporated herein by reference in its entirety).

Polyunsaturated fatty acids (PUFAs) are considered to be useful for nutritional, pharmaceutical, industrial, and other purposes. An expansive supply of PUFAs from natural sources and from chemical synthesis are not sufficient for commercial needs. Because a number of separate desaturase and elongase enzymes are required for fatty acid synthesis from linoleic acid (LA, 18:2 Δ 9, 12), common in most plant species, to the more saturated and longer chain PUFAs, engineering plant host cells for the expression of PUFAs such as EPA and DHA may require expression of five or six separate enzyme activities to achieve expression, at least for EPA and DHA. Additionally, for production of useable quantities of such PUFAs, additional engineering efforts may be required, for instance the down regulation of enzymes competing for substrate, engineering of higher enzyme activities such as by mutagenesis or targeting of enzymes to plastid organelles. Therefore it is of interest to obtain genetic material involved in PUFA biosynthesis from species that naturally produce these fatty acids and to express the isolated material alone or in combination in a heterologous system which can be manipulated to allow production of commercial quantities of PUFAs.

The discovery of a PUFA PKS system in marine bacteria such as Shewanella and Vibrio marinus (see U.S. Pat. No. 6,140,486, ibid.) provides a resource for new methods of commercial PUFA production. However, these marine bacteria have limitations which will ultimately restrict their usefulness on a commercial level. First, although U.S. Pat. No. 6,140,486 discloses that the marine bacteria PUFA PKS systems can be used to genetically modify plants, the marine bacteria naturally live and grow in cold marine environments and the enzyme systems of these bacteria do not function well above 30° C. In contrast, many crop plants, which are attractive targets for genetic manipulation using the PUFA PKS system, have normal growth conditions at temperatures above 30° C. and ranging to higher than 40° C. Therefore, the marine bacteria PUFA PKS system is not predicted to be readily adaptable to plant expression under normal growth conditions. Moreover, the marine bacteria PUFA PKS genes, being from a bacterial source, may not be compatible with the genomes of eukaryotic host cells, or at least may require significant adaptation to work in eukaryotic hosts. Additionally, the known marine bacteria PUFA PKS systems do not directly produce triglycerides, whereas direct production of triglycerides would be desirable because triglycerides are a lipid storage product in microorganisms and as a result can be accumulated at very high levels (e.g. up to 80-85% of cell weight) in microbial/plant cells (as opposed to a “structural” lipid product (e.g. phospholipids) which can generally only accumulate at low levels (e.g. less than 10-15% of cell weight at maximum)).

Therefore, there is a need in the art for other PUFA PKS systems having greater flexibility for commercial use.

SUMMARY OF THE INVENTION

One embodiment of the present invention relates to an isolated nucleic acid molecule comprising a nucleic acid sequence chosen from: (a) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof; (b) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof; (c) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of the amino acid sequence of (a), wherein the amino acid sequence has a biological activity of at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system; (d) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to the amino acid sequence of (b), wherein the amino acid sequence has a biological activity of at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system; and (e) a nucleic acid sequence that is fully complementary to the nucleic acid sequence of (a), (b), (c), or (d). In alternate aspects, the nucleic acid sequence encodes an amino acid sequence that is at least about 70% identical, or at least about 80% identical, or at least about 90% identical, or is identical to: (1) at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; and/or (2) a nucleic acid sequence encoding an amino acid sequence that is at least about 70% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32. In a preferred embodiment, the nucleic acid sequence encodes an amino acid sequence chosen from: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32 and/or biologically active fragments thereof. In one aspect, the nucleic acid sequence is chosen from: SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:12, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, SEQ ID NO:25, SEQ ID NO:27, SEQ ID NO:29, and SEQ ID NO:31.

Another embodiment of the present invention relates to a recombinant nucleic acid molecule comprising the nucleic acid molecule as described above, operatively linked to at least one transcription control sequence. In another embodiment, the present invention relates to a recombinant cell transfected with the recombinant nucleic acid molecule described directly above.

Yet another embodiment of the present invention relates to a genetically modified microorganism, wherein the microorganism expresses a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The at least one domain of the PUFA PKS system is encoded by a nucleic acid sequence chosen from: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by the screening method of the present invention; (c) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof, (d) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof; (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and, (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. In this embodiment, the microorganism is genetically modified to affect the activity of the PKS system. The screening method of the present invention referenced in (b) above comprises: (i) selecting a microorganism that produces at least one PUFA; and, (ii) identifying a microorganism from (i) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation in the fermentation medium, as compared to production of PUFAs by the microorganism under dissolved oxygen conditions of greater than 5% of saturation, and more preferably 10% of saturation, and more preferably greater than 15% of saturation, and more preferably greater than 20% of saturation in the fermentation medium.

In one aspect, the microorganism endogenously expresses a PKS system comprising the at least one domain of the PUFA PKS system, and wherein the genetic modification is in a nucleic acid sequence encoding the at least one domain of the PUFA PKS system. For example, the genetic modification can be in a nucleic acid sequence that encodes a domain having a biological activity of at least one of the following proteins: malonyl-CoA:ACP acyltransferase (MAT), β-keto acyl-ACP synthase (KS), ketoreductase (KR), acyltransferase (AT), FabA-like β-hydroxy acyl-ACP dehydrase (DH), phosphopantetheine transferase, chain length factor (CLF), acyl carrier protein (ACP), enoyl ACP-reductase (ER), an enzyme that catalyzes the synthesis of trans-2-decenoyl-ACP, an enzyme that catalyzes the reversible isomerization of trans-2-decenoyl-ACP to cis-3-decenoyl-ACP, and an enzyme that catalyzes the elongation of cis-3-decenoyl-ACP to cis-vaccenic acid. In one aspect, the genetic modification is in a nucleic acid sequence that encodes an amino acid sequence selected from the group consisting of: (a) an amino acid sequence that is at least about 70% identical, and preferably at least about 80% identical, and more preferably at least about 90% identical and more preferably identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and, (b) an amino acid sequence that is at least about 70% identical, and preferably at least about 80% identical, and more preferably at least about 90% identical and more preferably identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system.

In one aspect, the genetically modified microorganism is a Thraustochytrid, which can include, but is not limited to, a Thraustochytrid from a genus chosen from Schizochytrium and Thraustochytrium. In another aspect, the microorganism has been further genetically modified to recombinantly express at least one nucleic acid molecule encoding at least one biologically active domain from a bacterial PUFA PKS system, from a Type I PKS system, from a Type II PKS system, and/or from a modular PKS system.

In another aspect of this embodiment, the microorganism endogenously expresses a PUFA PKS system comprising the at least one biologically active domain of a PUFA PKS system, and wherein the genetic modification comprises expression of a recombinant nucleic acid molecule selected from the group consisting of a recombinant nucleic acid molecule encoding at least one biologically active domain from a second PKS system and a recombinant nucleic acid molecule encoding a protein that affects the activity of the PUFA PKS system. Preferably, the recombinant nucleic acid molecule comprises any one of the nucleic acid sequences described above.

In one aspect of this embodiment, the recombinant nucleic acid molecule encodes a phosphopantetheine transferase. In another aspect, the recombinant nucleic acid molecule comprises a nucleic acid sequence encoding at least one biologically active domain from a bacterial PUFA PKS system, from a type I PKS system, from a type II PKS system, and/or from a modular PKS system.

In another aspect of this embodiment, the microorganism is genetically modified by transfection with a recombinant nucleic acid molecule encoding the at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. Such a recombinant nucleic acid molecule can include any recombinant nucleic acid molecule comprising any of the nucleic acid sequences described above. In one aspect, the microorganism has been further genetically modified to recombinantly express at least one nucleic acid molecule encoding at least one biologically active domain from a bacterial PUFA PKS system, from a Type I PKS system, from a Type II PKS system, or from a modular PKS system.

Yet another embodiment of the present invention relates to a genetically modified plant, wherein the plant has been genetically modified to recombinantly express a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The domain can be encoded by any of the nucleic acid sequences described above. In one aspect, the plant has been further genetically modified to recombinantly express at least one nucleic acid molecule encoding at least one biologically active domain from a bacterial PUFA PKS system, from a Type I PKS system, from a Type II PKS system, and/from a modular PKS system.

Another embodiment of the present invention relates to a method to identify a microorganism that has a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The method includes the steps of: (a) selecting a microorganism that produces at least one PUFA; and, (b) identifying a microorganism from (a) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation in the fermentation medium, as compared to production of PUFAs by the microorganism under dissolved oxygen conditions of greater than 5% of saturation, more preferably 10% of saturation, more preferably greater than 15% of saturation and more preferably greater than 20% of saturation in the fermentation medium. A microorganism that produces at least one PUFA and has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation is identified as a candidate for containing a PUFA PKS system.

In one aspect of this embodiment, step (b) comprises identifying a microorganism from (a) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 2% of saturation, and more preferably under dissolved oxygen conditions of less than about 1% of saturation, and even more preferably under dissolved conditions of about 0% of saturation.

In another aspect of this embodiment, the microorganism selected in (a) has an ability to consume bacteria by phagocytosis. In another aspect, the microorganism selected in (a) has a simple fatty acid profile. In another aspect, the microorganism selected in (a) is a non-bacterial microorganism. In another aspect, the microorganism selected in (a) is a eukaryote. In another aspect, the microorganism selected in (a) is a member of the order Thraustochytriales. In another aspect, the microorganism selected in (a) has an ability to produce PUFAs at a temperature greater than about 15° C., and preferably greater than about 20° C., and more preferably greater than about 25° C., and even more preferably greater than about 30° C. In another aspect, the microorganism selected in (a) has an ability to produce bioactive compounds (e.g., lipids) of interest at greater than 5% of the dry weight of the organism, and more preferably greater than 10% of the dry weight of the organism. In yet another aspect, the microorganism selected in (a) contains greater than 30% of its total fatty acids as C14:0, C16:0 and C16:1 while also producing at least one long chain fatty acid with three or more unsaturated bonds, and preferably, the microorganism selected in (a) contains greater than 40% of its total fatty acids as C14:0, C16:0 and C16:1 while also producing at least one long chain fatty acid with three or more unsaturated bonds. In another aspect, the microorganism selected in (a) contains greater than 30% of its total fatty acids as C14:0, C16:0 and C16:1 while also producing at least one long chain fatty acid with four or more unsaturated bonds, and more preferably while also producing at least one long chain fatty acid with five or more unsaturated bonds.

In another aspect of this embodiment, the method further comprises step (c) of detecting whether the organism comprises a PUFA PKS system. In this aspect, the step of detecting can include detecting a nucleic acid sequence in the microorganism that hybridizes under stringent conditions with a nucleic acid sequence encoding an amino acid sequence from a Thraustochytrid PUFA PKS system. Alternatively, the step of detecting can include detecting a nucleic acid sequence in the organism that is amplified by oligonucleotide primers from a nucleic acid sequence from a Thaustochytrid PUFA PKS system.

Another embodiment of the present invention relates to a microorganism identified by the screening method described above, wherein the microorganism is genetically modified to regulate the production of molecules by the PUFA PKS system.

Yet another embodiment of the present invention relates to a method to produce a bioactive molecule that is produced by a polyketide synthase system. The method includes the step of culturing under conditions effective to produce the bioactive molecule a genetically modified organism that expresses a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The domain of the PUFA PKS system is encoded by any of the nucleic acid sequences described above.

In one aspect of this embodiment, the organism endogenously expresses a PKS system comprising the at least one domain of the PUFA PKS system, and the genetic modification is in a nucleic acid sequence encoding the at least one domain of the PUFA PKS system. For example, the genetic modification can change at least one product produced by the endogenous PKS system, as compared to a wild-type organism.

In another aspect of this embodiment, the organism endogenously expresses a PKS system comprising the at least one biologically active domain of the PUFA PKS system, and the genetic modification comprises transfection of the organism with a recombinant nucleic acid molecule selected from the group consisting of: a recombinant nucleic acid molecule encoding at least one biologically active domain from a second PKS system and a recombinant nucleic acid molecule encoding a protein that affects the activity of the PUFA PKS system. For example, the genetic modification can change at least one product produced by the endogenous PKS system, as compared to a wild-type organism.

In yet another aspect of this embodiment, the organism is genetically modified by transfection with a recombinant nucleic acid molecule encoding the at least one domain of the polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. In another aspect, the organism produces a polyunsaturated fatty acid (PUFA) profile that differs from the naturally occurring organism without a genetic modification. In another aspect, the organism endogenously expresses a non-bacterial PUFA PKS system, and wherein the genetic modification comprises substitution of a domain from a different PKS system for a nucleic acid sequence encoding at least one domain of the non-bacterial PUFA PKS system.

In yet another aspect, the organism endogenously expresses a non-bacterial PUFA PKS system that has been modified by transfecting the organism with a recombinant nucleic acid molecule encoding a protein that regulates the chain length of fatty acids produced by the PUFA PKS system. For example, the recombinant nucleic acid molecule encoding a protein that regulates the chain length of fatty acids can replace a nucleic acid sequence encoding a chain length factor in the non-bacterial PUFA PKS system. In another aspect, the protein that regulates the chain length of fatty acids produced by the PUFA PKS system is a chain length factor. In another aspect, the protein that regulates the chain length of fatty acids produced by the PUFA PKS system is a chain length factor that directs the synthesis of C20 units.

In one aspect, the organism expresses a non-bacterial PUFA PKS system comprising a genetic modification in a domain chosen from: a domain encoding FabA-like β-hydroxy acyl-ACP dehydrase (DH) domain and a domain encoding β-ketoacyl-ACP synthase (KS), wherein the modification alters the ratio of long chain fatty acids produced by the PUFA PKS system as compared to in the absence of the modification. In one aspect, the modification comprises substituting a DH domain that does not possess isomerization activity for a FabA-like β-hydroxy acyl-ACP dehydrase (DH) in the non-bacterial PUFA PKS system. In another aspect, the modification is selected from the group consisting of a deletion of all or a part of the domain, a substitution of a homologous domain from a different organism for the domain, and a mutation of the domain.

In another aspect, the organism expresses a PKS system and the genetic modification comprises substituting a FabA-like β-hydroxy acyl-ACP dehydrase (DH) domain from a PUFA PKS system for a DH domain that does not posses isomerization activity.

In another aspect, the organism expresses a non-bacterial PUFA PKS system comprising a modification in an enoyl-ACP reductase (ER) domain, wherein the modification results in the production of a different compound as compared to in the absence of the modification. For example, the modification can be selected from the group consisting of a deletion of all or a part of the ER domain, a substitution of an ER domain from a different organism for the ER domain, and a mutation of the ER domain.

In one aspect, the bioactive molecule produced by the present method can include, but is not limited to, an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti-Heliobactor pylori drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. In one aspect, the bioactive molecule is a polyunsaturated fatty acid (PUFA). In another aspect, the bioactive molecule is a molecule including carbon-carbon double bonds in the cis configuration. In another aspect, the bioactive molecule is a molecule including a double bond at every third carbon.

In one aspect of this embodiment, the organism is a microorganism, and in another aspect, the organism is a plant.

Another embodiment of the present invention relates to a method to produce a plant that has a polyunsaturated fatty acid (PUFA) profile that differs from the naturally occurring plant, comprising genetically modifying cells of the plant to express a PKS system comprising at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The domain of the PUFA PKS system is encoded by any of the nucleic acid sequences described above.

Yet another embodiment of the present invention relates to a method to modify an endproduct containing at least one fatty acid, comprising adding to the endproduct an oil produced by a recombinant host cell that expresses at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The domain of a PUFA PKS system is encoded by any of the nucleic acid sequences described above. In one aspect, the endproduct is selected from the group consisting of a dietary supplement, a food product, a pharmaceutical formulation, a humanized animal milk, and an infant formula. A pharmaceutical formulation can include, but is not limited to: an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti-Heliobactor pylori drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. In one aspect, the endproduct is used to treat a condition selected from the group consisting of: chronic inflammation, acute inflammation, gastrointestinal disorder, cancer, cachexia, cardiac restenosis, neurodegenerative disorder, degenerative disorder of the liver, blood lipid disorder, osteoporosis, osteoarthritis, autoimmune disease, preeclampsia, preterm birth, age related maculopathy, pulmonary disorder, and peroxisomal disorder.

Yet another embodiment of the present invention relates to a method to produce a humanized animal milk, comprising genetically modifying milk-producing cells of a milk-producing animal with at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The domain of the PUFA PKS system is encoded by any of the nucleic acid sequences described above.

Yet another embodiment of the present invention relates to a method produce a recombinant microbe, comprising genetically modifying microbial cells to express at least one recombinant nucleic acid molecule comprising a comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The domain of the PUFA PKS system is encoded by any of the nucleic acid sequences described above.

Yet another embodiment of the present invention relates to a recombinant host cell which has been modified to express a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PKS catalyzes both iterative and non-iterative enzymatic reactions. The PUFA PKS system comprises: (a) at least two enoyl ACP-reductase (ER) domains; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. In one aspect, the PUFA PKS system is a eukaryotic PUFA PKS system. In another aspect, the PUFA PKS system is an algal PUFA PKS system, and preferably a Thraustochytriales PUFA PKS system, which can include, but is not limited to, a Schizochytrium PUFA PKS system or a Thraustochytrium PUFA PKS system.

In this embodiment, the PUFA PKS system can be expressed in a prokaryotic host cell or in a eukaryotic host cell. In one aspect, the host cell is a plant cell. Accordingly, one embodiment of the invention is a method to produce a product containing at least one PUFA, comprising growing a plant comprising such a plant cell under conditions effective to produce the product. The host cell is a microbial cell and in this case, one embodiment of the present invention is a method to produce a product containing at least one PUFA, comprising culturing a culture containing such a microbial cell under conditions effective to produce the product. In one aspect, the PKS system catalyzes the direct production of triglycerides.

Yet another embodiment of the present invention relates to a genetically modified microorganism comprising a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PKS catalyzes both iterative and non-iterative enzymatic reactions. The PUFA PKS system comprises: (a) at least two enoyl ACP-reductase (ER) domains; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. The genetic modification affects the activity of the PUFA PKS system. In one aspect of this embodiment, the microorganism is a eukaryotic microorganism.

Yet another embodiment of the present invention relates to a recombinant host cell which has been modified to express a non-bacterial polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the non-bacterial PUFA PKS catalyzes both iterative and non-iterative enzymatic reactions. The non-bacterial PUFA PKS system comprises: (a) at least one enoyl ACP-reductase (ER) domain; (b) multiple acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graphical representation of the domain structure of the Schizochytrium PUFA PKS system.

FIG. 2 shows a comparison of PKS domains from Schizochytrium and Shewanella.

FIG. 3 shows a comparison of PKS domains from Schizochytrium and a related PKS system from Nostoc whose product is a long chain fatty acid that does not contain any double bonds.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to non-bacterial derived polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) systems, to genetically modified organisms comprising non-bacterial PUFA PKS systems, to methods of making and using such systems for the production of products of interest, including bioactive molecules, and to novel methods for identifying new eukaryotic microorganisms having such a PUFA PKS system. As used herein, a PUFA PKS system generally has the following identifying features: (1) it produces PUFAs as a natural product of the system; and (2) it comprises several multifunctional proteins assembled into a complex that conducts both iterative processing of the fatty acid chain as well non-iterative processing, including trans-cis isomerization and enoyl reduction reactions in selected cycles (See FIG. 1, for example).

More specifically, first, a PUFA PKS system that forms the basis of this invention produces polyunsaturated fatty acids (PUFAs) as products (i.e., an organism that endogenously (naturally) contains such a PKS system makes PUFAs using this system). The PUFAs referred to herein are preferably polyunsaturated fatty acids with a carbon chain length of at least 16 carbons, and more preferably at least 18 carbons, and more preferably at least 20 carbons, and more preferably 22 or more carbons, with at least 3 or more double bonds, and preferably 4 or more, and more preferably 5 or more, and even more preferably 6 or more double bonds, wherein all double bonds are in the cis configuration. It is an object of the present invention to find or create via genetic manipulation or manipulation of the endproduct, PKS systems which produce polyunsaturated fatty acids of desired chain length and with desired numbers of double bonds. Examples of PUFAs include, but are not limited to, DHA (docosahexaenoic acid (C22:6, ω-3)), DPA (docosapentaenoic acid (C22:5, ω-6)), and EPA (eicosapentaenoic acid (C20:5, ω-3)).

Second, the PUFA PKS system described herein incorporates both iterative and non-iterative reactions, which distinguish the system from previously described PKS systems (e.g., type I, type II or modular). More particularly, the PUFA PKS system described herein contains domains that appear to function during each cycle as well as those which appear to function during only some of the cycles. A key aspect of this may be related to the domains showing homology to the bacterial Fab A enzymes. For example, the Fab A enzyme of E. coli has been shown to possess two enzymatic activities. It possesses a dehydration activity in which a water molecule (H₂O) is abstracted from a carbon chain containing a hydroxy group, leaving a trans double bond in that carbon chain. In addition, it has an isomerase activity in which the trans double bond is converted to the cis configuration. This isomerization is accomplished in conjunction with a migration of the double bond position to adjacent carbons. In PKS (and FAS) systems, the main carbon chain is extended in 2 carbon increments. One can therefore predict the number of extension reactions required to produce the PUFA products of these PKS systems. For example, to produce DHA (C22:6, all cis) requires 10 extension reactions. Since there are only 6 double bonds in the end product, it means that during some of the reaction cycles, a double bond is retained (as a cis isomer), and in others, the double bond is reduced prior to the next extension.

Before the discovery of a PUFA PKS system in marine bacteria (see U.S. Pat. No. 6,140,486), PKS systems were not known to possess this combination of iterative and selective enzymatic reactions, and they were not thought of as being able to produce carbon-carbon double bonds in the cis configuration. However, the PUFA PKS system described by the present invention has the capacity to introduce cis double bonds and the capacity to vary the reaction sequence in the cycle.

Therefore, the present inventors propose to use these features of the PUFA PKS system to produce a range of bioactive molecules that could not be produced by the previously described (Type II, Type I and modular) PKS systems. These bioactive molecules include, but are limited to, polyunsaturated fatty acids (PUFAs), antibiotics or other bioactive compounds, many of which will be discussed below. For example, using the knowledge of the PUFA PKS gene structures described herein, any of a number of methods can be used to alter the PUFA PKS genes, or combine portions of these genes with other synthesis systems, including other PKS systems, such that new products are produced. The inherent ability of this particular type of system to do both iterative and selective reactions will enable this system to yield products that would not be found if similar methods were applied to other types of PKS systems.

In one embodiment, a PUFA PKS system according to the present invention comprises at least the following biologically active domains: (a) at least two enoyl ACP-reductase (ER) domains; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. The functions of these domains are generally individually known in the art and will be described in detail below with regard to the PUFA PKS system of the present invention.

In another embodiment, the PUFA PKS system comprises at least the following biologically active domains: (a) at least one enoyl ACP-reductase (ER) domain; (b) multiple acyl carrier protein (ACP) domains (at least four, and preferably at least five, and more preferably at least six, and even more preferably seven, eight, nine, or more than nine); (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. Preferably, such a PUFA PKS system is a non-bacterial PUFA-PKS system.

In one embodiment, a PUFA PKS system of the present invention is a non-bacterial PUFA PKS system. In other words, in one embodiment, the PUFA PKS system of the present invention is isolated from an organism that is not a bacteria, or is a homologue of or derived from a PUFA PKS system from an organism that is not a bacteria, such as a eukaryote or an archaebacterium. Eukaryotes are separated from prokaryotes based on the degree of differentiation of the cells. The higher group with more differentiation is called eukaryotic. The lower group with less differentiated cells is called prokaryotic. In general, prokaryotes do no possess a nuclear membrane, do not exhibit mitosis during cell division, have only one chromosome, their cytoplasm contains 70S ribosomes, they do not possess any mitochondria, endoplasmic reticulum, chloroplasts, lysosomes or golgi apparatus, their flagella (if present) consists of a single fibril. In contrast eukaryotes have a nuclear membrane, they do exhibit mitosis during cell division, they have many chromosomes, their cytoplasm contains 80S ribosomes, they do possess mitochondria, endoplasmic reticulum, chloroplasts (in algae), lysosomes and golgi apparatus, and their flagella (if present) consists of many fibrils. In general, bacteria are prokaryotes, while algae, fungi, protist, protozoa and higher plants are eukaryotes. The PUFA PKS systems of the marine bacteria (e.g., Shewanella and Vibrio marinus) are not the basis of the present invention, although the present invention does contemplate the use of domains from these bacterial PUFA PKS systems in conjunction with domains from the non-bacterial PUFA PKS systems of the present invention. For example, according to the present invention, genetically modified organisms can be produced which incorporate non-bacterial PUFA PKS functional domains with bacteria PUFA PKS functional domains, as well as PKS functional domains or proteins from other PKS systems (type I, type II, modular) or FAS systems.

Schizochytrium is a Thraustochytrid marine microorganism that accumulates large quantities of triacylglycerols rich in DHA and docosapentaenoic acid (DPA; 22:5 ω-6); e.g., 30% DHA+DPA by dry weight (Barclay et al., J. Appl. Phycol. 6, 123 (1994)). In eukaryotes that synthesize 20- and 22-carbon PUFAs by an elongation/desaturation pathway, the pools of 18-, 20- and 22-carbon intermediates are relatively large so that in vivo labeling experiments using [¹⁴C]-acetate reveal clear precursor-product kinetics for the predicted intermediates (Gellerman et al., Biochim. Biophys. Acta 573:23 (1979)). Furthermore, radiolabeled intermediates provided exogenously to such organisms are converted to the final PUFA products. The present inventors have shown that [1-¹⁴C]-acetate was rapidly taken up by Schizochytrium cells and incorporated into fatty acids, but at the shortest labeling time (1 min), DHA contained 31% of the label recovered in fatty acids, and this percentage remained essentially unchanged during the 10-15 min of [¹⁴C]-acetate incorporation and the subsequent 24 hours of culture growth (See Example 3). Similarly, DPA represented 10% of the label throughout the experiment. There is no evidence for a precursor-product relationship between 16- or 18-carbon fatty acids and the 22-carbon polyunsaturated fatty acids. These results are consistent with rapid synthesis of DHA from [¹⁴C]-acetate involving very small (possibly enzyme-bound) pools of intermediates. A cell-free homogenate derived from Schizochytrium cultures incorporated [1-¹⁴C]-malonyl-CoA into DHA, DPA, and saturated fatty acids. The same biosynthetic activities were retained by a 100,000×g supernatant fraction but were not present in the membrane pellet. Thus, DHA and DPA synthesis in Schizochytrium does not involve membrane-bound desaturases or fatty acid elongation enzymes like those described for other eukaryotes (Parker-Barnes et al., 2000, supra; Shanklin et al., 1998, supra). These fractionation data contrast with those obtained from the Shewanella enzymes (See Metz et al., 2001, supra) and may indicate use of a different (soluble) acyl acceptor molecule, such as CoA, by the Schizochytrium enzyme.

In copending U.S. application Ser. No. 09/231,899, a cDNA library from Schizochytrium was constructed and approximately 8,000 random clones (ESTs) were sequenced. Within this dataset, only one moderately expressed gene (0.3% of all sequences) was identified as a fatty acid desaturase, although a second putative desaturase was represented by a single clone (0.01%). By contrast, sequences that exhibited homology to 8 of the 11 domains of the Shewanella PKS genes shown in FIG. 2 were all identified at frequencies of 0.2-0.5%. In U.S. application Ser. No. 09/231,899, several cDNA clones showing homology to the Shewanella PKS genes were sequenced, and various clones were assembled into nucleic acid sequences representing two partial open reading frames and one complete open reading frame. Nucleotides 390-4443 of the cDNA sequence containing the first partial open reading frame described in U.S. application Ser. No. 09/231,899 (denoted therein as SEQ ID NO:69) match nucleotides 4677-8730 (plus the stop codon) of the sequence denoted herein as OrfA (SEQ ID NO:1). Nucleotides 1-4876 of the cDNA sequence containing the second partial open reading frame described in U.S. application Ser. No. 09/231,899 (denoted therein as SEQ ID NO:71) matches nucleotides 1311-6177 (plus the stop codon) of the sequence denoted herein as OrfB (SEQ ID NO:3). Nucleotides 145-4653 of the cDNA sequence containing the complete open reading frame described in U.S. application Ser. No. 09/231,899 (denoted therein as SEQ ID NO:76 and incorrectly designated as a partial open reading frame) match the entire sequence (plus the stop codon) of the sequence denoted herein as OrfC (SEQ ID NO:5).

Further sequencing of cDNA and genomic clones by the present inventors allowed the identification of the full-length genomic sequence of each of OrfA, OrfB and OrfC and the complete identification of the domains with homology to those in Shewanella (see FIG. 2). It is noted that in Schizochytrium, the genomic DNA and cDNA are identical, due to the lack of introns in the organism genome, to the best of the present inventors' knowledge. Therefore, reference to a nucleotide sequence from Schizochytrium can refer to genomic DNA or cDNA. Based on the comparison of the Schizochytrium PKS domains to Shewanella, clearly, the Schizochytrium genome encodes proteins that are highly similar to the proteins in Shewanella that are capable of catalyzing EPA synthesis. The proteins in Schizochytrium constitute a PUFA PKS system that catalyzes DHA and DPA synthesis. As discussed in detail herein, simple modification of the reaction scheme identified for Shewanella will allow for DHA synthesis in Schizochytrium. The homology between the prokaryotic Shewanella and eukaryotic Schizochytrium genes suggests that the PUFA PKS has undergone lateral gene transfer.

FIG. 1 is a graphical representation of the three open reading frames from the Schizochytrium PUFA PKS system, and includes the domain structure of this PUFA PKS system. As described in Example 1 below, the domain structure of each open reading frame is as follows:

Open Reading Frame A (OrfA):

The complete nucleotide sequence for OrfA is represented herein as SEQ ID NO:1. Nucleotides 4677-8730 of SEQ ID NO:1 correspond to nucleotides 390-4443 of the sequence denoted as SEQ ID NO:69 in U.S. application Ser. No. 09/231,899. Therefore, nucleotides 1-4676 of SEQ ID NO:1 represent additional sequence that was not disclosed in U.S. application Ser. No. 09/231,899. This novel region of SEQ ID NO:1 encodes the following domains in OrfA: (1) the ORFA-KS domain; (2) the ORFA-MAT domain; and (3) at least a portion of the ACP domain region (e.g., at least ACP domains 1-4). It is noted that nucleotides 1-389 of SEQ ID NO:69 in U.S. application Ser. No. 09/231,899 do not match with the 389 nucleotides that are upstream of position 4677 in SEQ ID NO:1 disclosed herein. Therefore, positions 1-389 of SEQ ID NO:69 in U.S. application Ser. No. 09/231,899 appear to be incorrectly placed next to nucleotides 390-4443 of that sequence. Most of these first 389 nucleotides (about positions 60-389) are a match with an upstream portion of OrfA (SEQ ID NO:1) of the present invention and therefore, it is believed that an error occurred in the effort to prepare the contig of the cDNA constructs in U.S. application Ser. No. 09/231,899. The region in which the alignment error occurred in U.S. application Ser. No. 09/231,899 is within the region of highly repetitive sequence (i.e., the ACP region, discussed below), which probably created some confusion in the assembly of that sequence from various cDNA clones.

OrfA is a 8730 nucleotide sequence (not including the stop codon) which encodes a 2910 amino acid sequence, represented herein as SEQ ID NO:2. Within OrfA are twelve domains: (a) one β-keto acyl-ACP synthase (KS) domain; (b) one malonyl-CoA:ACP acyltransferase (MAT) domain; (c) nine acyl carrier protein (ACP) domains; and (d) one ketoreductase (KR) domain.

The nucleotide sequence for OrfA has been deposited with GenBank as Accession No. AF378327 (amino acid sequence Accession No. AAK728879). OrfA was compared with known sequences in a standard BLAST search (BLAST 2.0 Basic BLAST homology search using blastp for amino acid searches, blastn for nucleic acid searches, and blastX for nucleic acid searches and searches of the translated amino acid sequence in all 6 open reading frames with standard default parameters, wherein the query sequence is filtered for low complexity regions by default (described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. & Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402, incorporated herein by reference in its entirety)). At the nucleic acid level, OrfA has no significant homology to any known nucleotide sequence. At the amino acid level, the sequences with the greatest degree of homology to ORFA were: Nostoc sp. 7120 heterocyst glycolipid synthase (Accession No. NC_(—)003272), which was 42% identical to ORFA over 1001 amino acid residues; and Moritella marinus (Vibrio marinus) ORF8 (Accession No. AB025342), which was 40% identical to ORFA over 993 amino acid residues.

The first domain in OrfA is a KS domain, also referred to herein as ORFA-KS. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 40 of SEQ ID NO:1 (OrfA) to an ending point of between about positions 1428 and 1500 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-KS domain is represented herein as SEQ ID NO:7 (positions 1-1500 of SEQ ID NO:1). The amino acid sequence containing the KS domain spans from a starting point of between about positions 1 and 14 of SEQ ID NO:2 (ORFA) to an ending point of between about positions 476 and 500 of SEQ ID NO:2. The amino acid sequence containing the ORFA-KS domain is represented herein as SEQ ID NO:8 (positions 1-500 of SEQ ID NO:2). It is noted that the ORFA-KS domain contains an active site motif: DXAC* (*acyl binding site C₂₁₅).

According to the present invention, a domain or protein having 3-keto acyl-ACP synthase (KS) biological activity (function) is characterized as the enzyme that carries out the initial step of the FAS (and PKS) elongation reaction cycle. The acyl group destined for elongation is linked to a cysteine residue at the active site of the enzyme by a thioester bond. In the multi-step reaction, the acyl-enzyme undergoes condensation with malonyl-ACP to form -keto acyl-ACP, CO₂ and free enzyme. The KS plays a key role in the elongation cycle and in many systems has been shown to possess greater substrate specificity than other enzymes of the reaction cycle. For example, E. coli has three distinct KS enzymes—each with its own particular role in the physiology of the organism (Magnuson et al., Microbiol. Rev. 57, 522 (1993)). The two KS domains of the PUFA-PKS systems could have distinct roles in the PUFA biosynthetic reaction sequence.

As a class of enzymes, KS's have been well characterized. The sequences of many verified KS genes are know, the active site motifs have been identified and the crystal structures of several have been determined. Proteins (or domains of proteins) can be readily identified as belonging to the KS family of enzymes by homology to known KS sequences.

The second domain in OrfA is a MAT domain, also referred to herein as ORFA-MAT. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1723 and 1798 of SEQ ID NO:1 (OrfA) to an ending point of between about positions 2805 and 3000 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-MAT domain is represented herein as SEQ ID NO:9 (positions 1723-3000 of SEQ ID NO:1). The amino acid sequence containing the MAT domain spans from a starting point of between about positions 575 and 600 of SEQ ID NO:2 (ORFA) to an ending point of between about positions 935 and 1000 of SEQ ID NO:2. The amino acid sequence containing the ORFA-MAT domain is represented herein as SEQ ID NO:10 (positions 575-1000 of SEQ ID NO:2). It is noted that the ORFA-MAT domain contains an active site motif: GHS*XG (*acyl binding site S₇₀₆), represented herein as SEQ ID NO:11.

According to the present invention, a domain or protein having malonyl-CoA:ACP acyltransferase (MAT) biological activity (function) is characterized as one that transfers the malonyl moiety from malonyl-CoA to ACP. In addition to the active site motif (GxSxG), these enzymes possess an extended Motif® and Q amino acids in key positions) that identifies them as MAT enzymes (in contrast to the AT domain of Schizochytrium Orf B). In some PKS systems (but not the PUFA PKS domain) MAT domains will preferentially load methyl- or ethyl-malonate on to the ACP group (from the corresponding CoA ester), thereby introducing branches into the linear carbon chain. MAT domains can be recognized by their homology to known MAT sequences and by their extended motif structure.

Domains 3-11 of OrfA are nine tandem ACP domains, also referred to herein as ORFA-ACP (the first domain in the sequence is ORFA-ACP1, the second domain is ORFA-ACP2, the third domain is ORFA-ACP3, etc.). The first ACP domain, ORFA-ACP1, is contained within the nucleotide sequence spanning from about position 3343 to about position 3600 of SEQ ID NO:1 (OrfA). The nucleotide sequence containing the sequence encoding the ORFA-ACP1 domain is represented herein as SEQ ID NO:12 (positions 3343-3600 of SEQ ID NO:1). The amino acid sequence containing the first ACP domain spans from about position 1115 to about position 1200 of SEQ ID NO:2. The amino acid sequence containing the ORFA-ACP1 domain is represented herein as SEQ ID NO:13 (positions 1115-1200 of SEQ ID NO:2). It is noted that the ORFA-ACP1 domain contains an active site motif: LGIDS* (*pantetheine binding motif S₁₁₅₇), represented herein by SEQ ID NO:14.

The nucleotide and amino acid sequences of all nine ACP domains are highly conserved and therefore, the sequence for each domain is not represented herein by an individual sequence identifier. However, based on the information disclosed herein, one of skill in the art can readily determine the sequence containing each of the other eight ACP domains (see discussion below).

All nine ACP domains together span a region of OrfA of from about position 3283 to about position 6288 of SEQ ID NO:1, which corresponds to amino acid positions of from about 1095 to about 2096 of SEQ ID NO:2. The nucleotide sequence for the entire ACP region containing all nine domains is represented herein as SEQ ID NO:16. The region represented by SEQ ID NO:16 includes the linker segments between individual ACP domains. The repeat interval for the nine domains is approximately every 330 nucleotides of SEQ ID NO:16 (the actual number of amino acids measured between adjacent active site serines ranges from 104 to 116 amino acids). Each of the nine ACP domains contains a pantetheine binding motif LGIDS* (represented herein by SEQ ID NO:14), wherein S* is the pantetheine binding site serine(S). The pantetheine binding site serine (S) is located near the center of each ACP domain sequence. At each end of the ACP domain region and between each ACP domain is a region that is highly enriched for proline (P) and alanine (A), which is believed to be a linker region. For example, between ACP domains 1 and 2 is the sequence: APAPVKAAAPAAPVASAPAPA, represented herein as SEQ ID NO:15. The locations of the active site serine residues (i.e., the pantetheine binding site) for each of the nine ACP domains, with respect to the amino acid sequence of SEQ ID NO:2, are as follows: ACP1=S₁₁₅₇; ACP2=S₁₂₆₆; ACP3=S₁₃₇₇; ACP4=S₁₄₈₈; ACP5=S₁₆₀₄; ACP6=S₁₇₁₅; ACP7=S₁₈₁₉; ACP8=S₁₉₃₀; and ACP9=S₂₀₃₄. Given that the average size of an ACP domain is about 85 amino acids, excluding the linker, and about 110 amino acids including the linker, with the active site serine being approximately in the center of the domain, one of skill in the art can readily determine the positions of each of the nine ACP domains in OrfA.

According to the present invention, a domain or protein having acyl carrier protein (ACP) biological activity (function) is characterized as being small polypeptides (typically, 80 to 100 amino acids long), that function as carriers for growing fatty acyl chains via a thioester linkage to a covalently bound co-factor of the protein. They occur as separate units or as domains within larger proteins. ACPs are converted from inactive apo-forms to functional holo-forms by transfer of the phosphopantetheinyl moeity of CoA to a highly conserved serine residue of the ACP. Acyl groups are attached to ACP by a thioester linkage at the free terminus of the phosphopantetheinyl moiety. ACPs can be identified by labeling with radioactive pantetheine and by sequence homology to known ACPs. The presence of variations of the above mentioned motif (LGIDS*) is also a signature of an ACP.

Domain 12 in OrfA is a KR domain, also referred to herein as ORFA-KR. This domain is contained within the nucleotide sequence spanning from a starting point of about position 6598 of SEQ ID NO:1 to an ending point of about position 8730 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-KR domain is represented herein as SEQ ID NO:17 (positions 6598-8730 of SEQ ID NO:1). The amino acid sequence containing the KR domain spans from a starting point of about position 2200 of SEQ ID NO:2 (ORFA) to an ending point of about position 2910 of SEQ ID NO:2. The amino acid sequence containing the ORFA-KR domain is represented herein as SEQ ID NO:18 (positions 2200-2910 of SEQ ID NO:2). Within the KR domain is a core region with homology to short chain aldehyde-dehydrogenases (KR is a member of this family). This core region spans from about position 7198 to about position 7500 of SEQ ID NO:1, which corresponds to amino acid positions 2400-2500 of SEQ ID NO:2.

According to the present invention, a domain or protein having ketoreductase activity, also referred to as 3-ketoacyl-ACP reductase (KR) biological activity (function), is characterized as one that catalyzes the pyridine-nucleotide-dependent reduction of 3-keto acyl forms of ACP. It is the first reductive step in the de novo fatty acid biosynthesis elongation cycle and a reaction often performed in polyketide biosynthesis. Significant sequence similarity is observed with one family of enoyl ACP reductases (ER), the other reductase of FAS (but not the ER family present in the PUFA PKS system), and the short-chain alcohol dehydrogenase family. Pfam analysis of the PUFA PKS region indicated above reveals the homology to the short-chain alcohol dehydrogenase family in the core region. Blast analysis of the same region reveals matches in the core area to known KR enzymes as well as an extended region of homology to domains from the other characterized PUFA PKS systems.

Open Reading Frame B (OrfB):

The complete nucleotide sequence for OrfB is represented herein as SEQ ID NO:3. Nucleotides 1311-4242 and 4244-6177 of SEQ ID NO:3 correspond to nucleotides 1-2932 and 2934-4867 of the sequence denoted as SEQ ID NO:71 in U.S. application Ser. No. 09/231,899 (The cDNA sequence in U.S. application Ser. No. 09/231,899 contains about 345 additional nucleotides beyond the stop codon, including a polyA tail). Therefore, nucleotides 1-1310 of SEQ ID NO:1 represent additional sequence that was not disclosed in U.S. application Ser. No. 09/231,899. This novel region of SEQ ID NO:3 contains most of the KS domain encoded by OrfB.

OrfB is a 6177 nucleotide sequence (not including the stop codon) which encodes a 2059 amino acid sequence, represented herein as SEQ ID NO:4. Within OrfB are four domains: (a) one β-keto acyl-ACP synthase (KS) domain; (b) one chain length factor (CLF) domain; (c) one acyl transferase (AT) domain; and, (d) one enoyl ACP-reductase (ER) domain.

The nucleotide sequence for OrfB has been deposited with GenBank as Accession No. AF378328 (amino acid sequence Accession No. AAK728880). OrfB was compared with known sequences in a standard BLAST search as described above. At the nucleic acid level, OrfB has no significant homology to any known nucleotide sequence. At the amino acid level, the sequences with the greatest degree of homology to ORFB were: Shewanella sp. hypothetical protein (Accession No. U73935), which was 53% identical to ORFB over 458 amino acid residues; Moritella marinus (Vibrio marinus) ORF11 (Accession No. AB025342), which was 53% identical to ORFB over 460 amino acid residues; Photobacterium profundum omega-3 polyunsaturated fatty acid synthase PfaD (Accession No. AF409100), which was 52% identical to ORFB over 457 amino acid residues; and Nostoc sp. 7120 hypothetical protein (Accession No. NC_(—)003272), which was 53% identical to ORFB over 430 amino acid residues.

The first domain in OrfB is a KS domain, also referred to herein as ORFB-KS. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 43 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 1332 and 1350 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-KS domain is represented herein as SEQ ID NO:19 (positions 1-1350 of SEQ ID NO:3). The amino acid sequence containing the KS domain spans from a starting point of between about positions 1 and 15 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 444 and 450 of SEQ ID NO:4. The amino acid sequence containing the ORFB-KS domain is represented herein as SEQ ID NO:20 (positions 1-450 of SEQ ID NO:4). It is noted that the ORFB-KS domain contains an active site motif: DXAC* (*acyl binding site C₁₉₆). KS biological activity and methods of identifying proteins or domains having such activity is described above.

The second domain in OrfB is a CLF domain, also referred to herein as ORFB-CLF. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1378 and 1402 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 2682 and 2700 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-CLF domain is represented herein as SEQ ID NO:21 (positions 1378-2700 of SEQ ID NO:3). The amino acid sequence containing the CLF domain spans from a starting point of between about positions 460 and 468 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 894 and 900 of SEQ ID NO:4. The amino acid sequence containing the ORFB-CLF domain is represented herein as SEQ ID NO:22 (positions 460-900 of SEQ ID NO:4). It is noted that the ORFB-CLF domain contains a KS active site motif without the acyl-binding cysteine.

According to the present invention, a domain or protein is referred to as a chain length factor (CLF) based on the following rationale. The CLF was originally described as characteristic of Type II (dissociated enzymes) PKS systems and was hypothesized to play a role in determining the number of elongation cycles, and hence the chain length, of the end product. CLF amino acid sequences show homology to KS domains (and are thought to form heterodimers with a KS protein), but they lack the active site cysteine. CLF's role in PKS systems is currently controversial. New evidence (C. Bisang et al., Nature 401, 502 (1999)) suggests a role in priming (providing the initial acyl group to be elongated) the PKS systems. In this role the CLF domain is thought to decarboxylate malonate (as malonyl-ACP), thus forming an acetate group that can be transferred to the KS active site. This acetate therefore acts as the ‘priming’ molecule that can undergo the initial elongation (condensation) reaction. Homologues of the Type II CLF have been identified as ‘loading’ domains in some modular PKS systems. A domain with the sequence features of the CLF is found in all currently identified PUFA PKS systems and in each case is found as part of a multidomain protein.

The third domain in OrfB is an AT domain, also referred to herein as ORFB-AT. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 2701 and 3598 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 3975 and 4200 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-AT domain is represented herein as SEQ ID NO:23 (positions 2701-4200 of SEQ ID NO:3). The amino acid sequence containing the AT domain spans from a starting point of between about positions 901 and 1200 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 1325 and 1400 of SEQ ID NO:4. The amino acid sequence containing the ORFB-AT domain is represented herein as SEQ ID NO:24 (positions 901-1400 of SEQ ID NO:4). It is noted that the ORFB-AT domain contains an active site motif of GxS*xG (*acyl binding site S₁₁₄₀) that is characteristic of acyltransferse (AT) proteins.

An “acyltransferase” or “AT” refers to a general class of enzymes that can carry out a number of distinct acyl transfer reactions. The Schizochytrium domain shows good homology to a domain present in all of the other PUFA PKS systems currently examined and very weak homology to some acyltransferases whose specific functions have been identified (e.g. to malonyl-CoA:ACP acyltransferase, MAT). In spite of the weak homology to MAT, this AT domain is not believed to function as a MAT because it does not possess an extended motif structure characteristic of such enzymes (see MAT domain description, above). For the purposes of this disclosure, the functions of the AT domain in a PUFA PKS system include, but are not limited to: transfer of the fatty acyl group from the ORFA ACP domain(s) to water (i.e. a thioesterase—releasing the fatty acyl group as a free fatty acid), transfer of a fatty acyl group to an acceptor such as CoA, transfer of the acyl group among the various ACP domains, or transfer of the fatty acyl group to a lipophilic acceptor molecule (e.g. to lysophosphadic acid).

The fourth domain in OrfB is an ER domain, also referred to herein as ORFB-ER. This domain is contained within the nucleotide sequence spanning from a starting point of about position 4648 of SEQ ID NO:3 (OrfB) to an ending point of about position 6177 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-ER domain is represented herein as SEQ ID NO:25 (positions 4648-6177 of SEQ ID NO:3). The amino acid sequence containing the ER domain spans from a starting point of about position 1550 of SEQ ID NO:4 (ORFB) to an ending point of about position 2059 of SEQ ID NO:4. The amino acid sequence containing the ORFB-ER domain is represented herein as SEQ ID NO:26 (positions 1550-2059 of SEQ ID NO:4).

According to the present invention, this domain has enoyl reductase (ER) biological activity. The ER enzyme reduces the trans-double bond (introduced by the DH activity) in the fatty acyl-ACP, resulting in fully saturating those carbons. The ER domain in the PUFA-PKS shows homology to a newly characterized family of ER enzymes (Heath et al., Nature 406, 145 (2000)). Heath and Rock identified this new class of ER enzymes by cloning a gene of interest from Streptococcus pneumoniae, purifying a protein expressed from that gene, and showing that it had ER activity in an in vitro assay. The sequence of the Schizochytrium ER domain of OrfB shows homology to the S. pneumoniae ER protein. All of the PUFA PKS systems currently examined contain at least one domain with very high sequence homology to the Schizochytrium ER domain. The Schizochytrium PUFA PKS system contains two ER domains (one on OrfB and one on OrfC).

Open Reading Frame C (OrfC):

The complete nucleotide sequence for OrfC is represented herein as SEQ ID NO:5. Nucleotides 1-4506 of SEQ ID NO:5 (i.e., the entire open reading frame sequence, not including the stop codon) correspond to nucleotides 145-2768, 2770-2805, 2807-2817, and 2819-4653 of the sequence denoted as SEQ ID NO:76 in U.S. application Ser. No. 09/231,899 (The cDNA sequence in U.S. application Ser. No. 09/231,899 contains about 144 nucleotides upstream of the start codon for OrfC and about 110 nucleotides beyond the stop codon, including a polyA tail). OrfC is a 4506 nucleotide sequence (not including the stop codon) which encodes a 1502 amino acid sequence, represented herein as SEQ ID NO:6. Within OrfC are three domains: (a) two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; and (b) one enoyl ACP-reductase (ER) domain.

The nucleotide sequence for OrfC has been deposited with GenBank as Accession No. AF378329 (amino acid sequence Accession No. AAK728881). OrfC was compared with known sequences in a standard BLAST search as described above. At the nucleic acid level, OrfC has no significant homology to any known nucleotide sequence. At the amino acid level (Blastp), the sequences with the greatest degree of homology to ORFC were: Moritella marinus (Vibrio marinus) ORF11 (Accession No. ABO25342), which is 45% identical to ORFC over 514 amino acid residues, Shewanella sp. hypothetical protein 8 (Accession No. U73935), which is 49% identical to ORFC over 447 amino acid residues, Nostoc sp. hypothetical protein (Accession No. NC_(—)003272), which is 49% identical to ORFC over 430 amino acid residues, and Shewanella sp. hypothetical protein 7 (Accession No. U73935), which is 37% identical to ORFC over 930 amino acid residues.

The first domain in OrfC is a DH domain, also referred to herein as ORFC-DH1. This is one of two DH domains in OrfC, and therefore is designated DH1. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 778 of SEQ ID NO:5 (OrfC) to an ending point of between about positions 1233 and 1350 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-DH1 domain is represented herein as SEQ ID NO:27 (positions 1-1350 of SEQ ID NO:5). The amino acid sequence containing the DH1 domain spans from a starting point of between about positions 1 and 260 of SEQ ID NO:6 (ORFC) to an ending point of between about positions 411 and 450 of SEQ ID NO:6. The amino acid sequence containing the ORFC-DH1 domain is represented herein as SEQ ID NO:28 (positions 1-450 of SEQ ID NO:6).

The characteristics of both the DH domains (see below for DH 2) in the PUFA PKS systems have been described in the preceding sections. This class of enzyme removes HOH from a β-keto acyl-ACP and leaves a trans double bond in the carbon chain. The DH domains of the PUFA PKS systems show homology to bacterial DH enzymes associated with their FAS systems (rather than to the DH domains of other PKS systems). A subset of bacterial DH's, the FabA-like DH's, possesses cis-trans isomerase activity (Heath et al., J. Biol. Chem., 271, 27795 (1996)). It is the homologies to the FabA-like DH's that indicate that one or both of the DH domains is responsible for insertion of the cis double bonds in the PUFA PKS products.

The second domain in OrfC is a DH domain, also referred to herein as ORFC-DH2. This is the second of two DH domains in OrfC, and therefore is designated DH2. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1351 and 2437 of SEQ ID NO:5 (OrfC) to an ending point of between about positions 2607 and 2847 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-DH2 domain is represented herein as SEQ ID NO:29 (positions 1351-2847 of SEQ ID NO:5). The amino acid sequence containing the DH2 domain spans from a starting point of between about positions 451 and 813 of SEQ ID NO:6 (ORFC) to an ending point of between about positions 869 and 949 of SEQ ID NO:6. The amino acid sequence containing the ORFC-DH2 domain is represented herein as SEQ ID NO:30 (positions 451-949 of SEQ ID NO:6). DH biological activity has been described above.

The third domain in OrfC is an ER domain, also referred to herein as ORFC-ER. This domain is contained within the nucleotide sequence spanning from a starting point of about position 2995 of SEQ ID NO:5 (OrfC) to an ending point of about position 4506 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-ER domain is represented herein as SEQ ID NO:31 (positions 2995-4506 of SEQ ID NO:5). The amino acid sequence containing the ER domain spans from a starting point of about position 999 of SEQ ID NO:6 (ORFC) to an ending point of about position 1502 of SEQ ID NO:6. The amino acid sequence containing the ORFC-ER domain is represented herein as SEQ ID NO:32 (positions 999-1502 of SEQ ID NO:6). ER biological activity has been described above.

One embodiment of the present invention relates to an isolated nucleic acid molecule comprising a nucleic acid sequence from a non-bacterial PUFA PKS system, a homologue thereof, a fragment thereof, and/or a nucleic acid sequence that is complementary to any of such nucleic acid sequences. In one aspect, the present invention relates to an isolated nucleic acid molecule comprising a nucleic acid sequence selected from the group consisting of: (a) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof, (b) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof; (c) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of said amino acid sequence of (a), wherein said amino acid sequence has a biological activity of at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system; (d) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to said amino acid sequence of (b), wherein said amino acid sequence has a biological activity of at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system; or (e) a nucleic acid sequence that is fully complementary to the nucleic acid sequence of (a), (b), (c), or (d). In a further embodiment, nucleic acid sequences including a sequence encoding the active site domains or other functional motifs described above for several of the PUFA PKS domains are encompassed by the invention.

According to the present invention, an amino acid sequence that has a biological activity of at least one domain of a PUFA PKS system is an amino acid sequence that has the biological activity of at least one domain of the PUFA PKS system described in detail herein, as exemplified by the Schizochytrium PUFA PKS system. The biological activities of the various domains within the Schizochytrium PUFA PKS system have been described in detail above. Therefore, an isolated nucleic acid molecule of the present invention can encode the translation product of any PUFA PKS open reading frame, PUFA PKS domain, biologically active fragment thereof, or any homologue of a naturally occurring PUFA PKS open reading frame or domain which has biological activity. A homologue of given protein or domain is a protein or polypeptide that has an amino acid sequence which differs from the naturally occurring reference amino acid sequence (i.e., of the reference protein or domain) in that at least one or a few, but not limited to one or a few, amino acids have been deleted (e.g., a truncated version of the protein, such as a peptide or fragment), inserted, inverted, substituted and/or derivatized (e.g., by glycosylation, phosphorylation, acetylation, myristoylation, prenylation, palmitation, amidation and/or addition of glycosylphosphatidyl inositol). Preferred homologues of a PUFA PKS protein or domain are described in detail below. It is noted that homologues can include synthetically produced homologues, naturally occurring allelic variants of a given protein or domain, or homologous sequences from organisms other than the organism from which the reference sequence was derived.

In general, the biological activity or biological action of a protein or domain refers to any function(s) exhibited or performed by the protein or domain that is ascribed to the naturally occurring form of the protein or domain as measured or observed in vivo (i.e., in the natural physiological environment of the protein) or in vitro (i.e., under laboratory conditions). Biological activities of PUFA PKS systems and the individual proteins/domains that make up a PUFA PKS system have been described in detail elsewhere herein. Modifications of a protein or domain, such as in a homologue or mimetic (discussed below), may result in proteins or domains having the same biological activity as the naturally occurring protein or domain, or in proteins or domains having decreased or increased biological activity as compared to the naturally occurring protein or domain. Modifications which result in a decrease in expression or a decrease in the activity of the protein or domain, can be referred to as inactivation (complete or partial), down-regulation, or decreased action of a protein or domain. Similarly, modifications which result in an increase in expression or an increase in the activity of the protein or domain, can be referred to as amplification, overproduction, activation, enhancement, up-regulation or increased action of a protein or domain. A functional domain of a PUFA PKS system is a domain (i.e., a domain can be a portion of a protein) that is capable of performing a biological function (i.e., has biological activity).

In accordance with the present invention, an isolated nucleic acid molecule is a nucleic acid molecule that has been removed from its natural milieu (i.e., that has been subject to human manipulation), its natural milieu being the genome or chromosome in which the nucleic acid molecule is found in nature. As such, “isolated” does not necessarily reflect the extent to which the nucleic acid molecule has been purified, but indicates that the molecule does not include an entire genome or an entire chromosome in which the nucleic acid molecule is found in nature. An isolated nucleic acid molecule can include a gene. An isolated nucleic acid molecule that includes a gene is not a fragment of a chromosome that includes such gene, but rather includes the coding region and regulatory regions associated with the gene, but no additional genes naturally found on the same chromosome. An isolated nucleic acid molecule can also include a specified nucleic acid sequence flanked by (i.e., at the 5′ and/or the 3′ end of the sequence) additional nucleic acids that do not normally flank the specified nucleic acid sequence in nature (i.e., heterologous sequences). Isolated nucleic acid molecule can include DNA, RNA (e.g., mRNA), or derivatives of either DNA or RNA (e.g., cDNA). Although the phrase “nucleic acid molecule” primarily refers to the physical nucleic acid molecule and the phrase “nucleic acid sequence” primarily refers to the sequence of nucleotides on the nucleic acid molecule, the two phrases can be used interchangeably, especially with respect to a nucleic acid molecule, or a nucleic acid sequence, being capable of encoding a protein or domain of a protein.

Preferably, an isolated nucleic acid molecule of the present invention is produced using recombinant DNA technology (e.g., polymerase chain reaction (PCR) amplification, cloning) or chemical synthesis. Isolated nucleic acid molecules include natural nucleic acid molecules and homologues thereof, including, but not limited to, natural allelic variants and modified nucleic acid molecules in which nucleotides have been inserted, deleted, substituted, and/or inverted in such a manner that such modifications provide the desired effect on PUFA PKS system biological activity as described herein. Protein homologues (e.g., proteins encoded by nucleic acid homologues) have been discussed in detail above.

A nucleic acid molecule homologue can be produced using a number of methods known to those skilled in the art (see, for example, Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Labs Press, 1989). For example, nucleic acid molecules can be modified using a variety of techniques including, but not limited to, classic mutagenesis techniques and recombinant DNA techniques, such as site-directed mutagenesis, chemical treatment of a nucleic acid molecule to induce mutations, restriction enzyme cleavage of a nucleic acid fragment, ligation of nucleic acid fragments, PCR amplification and/or mutagenesis of selected regions of a nucleic acid sequence, synthesis of oligonucleotide mixtures and ligation of mixture groups to “build” a mixture of nucleic acid molecules and combinations thereof. Nucleic acid molecule homologues can be selected from a mixture of modified nucleic acids by screening for the function of the protein encoded by the nucleic acid and/or by hybridization with a wild-type gene.

The minimum size of a nucleic acid molecule of the present invention is a size sufficient to form a probe or oligonucleotide primer that is capable of forming a stable hybrid (e.g., under moderate, high or very high stringency conditions) with the complementary sequence of a nucleic acid molecule useful in the present invention, or of a size sufficient to encode an amino acid sequence having a biological activity of at least one domain of a PUFA PKS system according to the present invention. As such, the size of the nucleic acid molecule encoding such a protein can be dependent on nucleic acid composition and percent homology or identity between the nucleic acid molecule and complementary sequence as well as upon hybridization conditions per se (e.g., temperature, salt concentration, and formamide concentration). The minimal size of a nucleic acid molecule that is used as an oligonucleotide primer or as a probe is typically at least about 12 to about 15 nucleotides in length if the nucleic acid molecules are GC-rich and at least about 15 to about 18 bases in length if they are AT-rich. There is no limit, other than a practical limit, on the maximal size of a nucleic acid molecule of the present invention, in that the nucleic acid molecule can include a sequence sufficient to encode a biologically active fragment of a domain of a PUFA PKS system, an entire domain of a PUFA PKS system, several domains within an open reading frame (Orf) of a PUFA PKS system, an entire Orf of a PUFA PKS system, or more than one Orf of a PUFA PKS system.

In one embodiment of the present invention, an isolated nucleic acid molecule comprises or consists essentially of a nucleic acid sequence selected from the group of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, or biologically active fragments thereof. In one aspect, the nucleic acid sequence is selected from the group of: SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:12, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, SEQ ID NO:25, SEQ ID NO:27, SEQ ID NO:29, and SEQ ID NO:31. In one embodiment of the present invention, any of the above-described PUFA PKS amino acid sequences, as well as homologues of such sequences, can be produced with from at least one, and up to about 20, additional heterologous amino acids flanking each of the C- and/or N-terminal end of the given amino acid sequence. The resulting protein or polypeptide can be referred to as “consisting essentially of” a given amino acid sequence. According to the present invention, the heterologous amino acids are a sequence of amino acids that are not naturally found (i.e., not found in nature, in vivo) flanking the given amino acid sequence or which would not be encoded by the nucleotides that flank the naturally occurring nucleic acid sequence encoding the given amino acid sequence as it occurs in the gene, if such nucleotides in the naturally occurring sequence were translated using standard codon usage for the organism from which the given amino acid sequence is derived. Similarly, the phrase “consisting essentially of”, when used with reference to a nucleic acid sequence herein, refers to a nucleic acid sequence encoding a given amino acid sequence that can be flanked by from at least one, and up to as many as about 60, additional heterologous nucleotides at each of the 5′ and/or the 3′ end of the nucleic acid sequence encoding the given amino acid sequence. The heterologous nucleotides are not naturally found (i.e., not found in nature, in vivo) flanking the nucleic acid sequence encoding the given amino acid sequence as it occurs in the natural gene.

The present invention also includes an isolated nucleic acid molecule comprising a nucleic acid sequence encoding an amino acid sequence having a biological activity of at least one domain of a PUFA PKS system. In one aspect, such a nucleic acid sequence encodes a homologue of any of the Schizochytrium PUFA PKS ORFs or domains, including: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, or SEQ ID NO:32, wherein the homologue has a biological activity of at least one domain of a PUFA PKS system as described previously herein.

In one aspect of the invention, a homologue of a Schizochytrium PUFA PKS protein or domain encompassed by the present invention comprises an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence chosen from: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein said amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. In a further aspect, the amino acid sequence of the homologue is at least about 60% identical to at least about 600 consecutive amino acids, and more preferably to at least about 700 consecutive amino acids, and more preferably to at least about 800 consecutive amino acids, and more preferably to at least about 900 consecutive amino acids, and more preferably to at least about 1000 consecutive amino acids, and more preferably to at least about 1100 consecutive amino acids, and more preferably to at least about 1200 consecutive amino acids, and more preferably to at least about 1300 consecutive amino acids, and more preferably to at least about 1400 consecutive amino acids, and more preferably to at least about 1500 consecutive amino acids of any of SEQ ID NO:2, SEQ ID NO:4 and SEQ ID NO:6, or to the full length of SEQ ID NO:6. In a further aspect, the amino acid sequence of the homologue is at least about 60% identical to at least about 1600 consecutive amino acids, and more preferably to at least about 1700 consecutive amino acids, and more preferably to at least about 1800 consecutive amino acids, and more preferably to at least about 1900 consecutive amino acids, and more preferably to at least about 2000 consecutive amino acids of any of SEQ ID NO:2 or SEQ ID NO:4, or to the full length of SEQ ID NO:4. In a further aspect, the amino acid sequence of the homologue is at least about 60% identical to at least about 2100 consecutive amino acids, and more preferably to at least about 2200 consecutive amino acids, and more preferably to at least about 2300 consecutive amino acids, and more preferably to at least about 2400 consecutive amino acids, and more preferably to at least about 2500 consecutive amino acids, and more preferably to at least about 2600 consecutive amino acids, and more preferably to at least about 2700 consecutive amino acids, and more preferably to at least about 2800 consecutive amino acids, and even more preferably, to the full length of SEQ ID NO:2.

In another aspect, a homologue of a Schizochytrium PUFA PKS protein or domain encompassed by the present invention comprises an amino acid sequence that is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical to an amino acid sequence chosen from: SEQ ID NO:2, SEQ ID NO:4, or SEQ ID NO:6, over any of the consecutive amino acid lengths described in the paragraph above, wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system.

In one aspect of the invention, a homologue of a Schizochytrium PUFA PKS protein or domain encompassed by the present invention comprises an amino acid sequence that is at least about 60% identical to an amino acid sequence chosen from: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, or SEQ ID NO:32, wherein said amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. In a further aspect, the amino acid sequence of the homologue is at least about 65% identical, and more preferably at least about 70% identical, and more preferably at least about 75% identical, and more preferably at least about 80% identical, and more preferably at least about 85% identical, and more preferably at least about 90% identical, and more preferably at least about 95% identical, and more preferably at least about 96% identical, and more preferably at least about 97% identical, and more preferably at least about 98% identical, and more preferably at least about 99% identical to an amino acid sequence chosen from: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system.

According to the present invention, the term “contiguous” or “consecutive”, with regard to nucleic acid or amino acid sequences described herein, means to be connected in an unbroken sequence. For example, for a first sequence to comprise 30 contiguous (or consecutive) amino acids of a second sequence, means that the first sequence includes an unbroken sequence of 30 amino acid residues that is 100% identical to an unbroken sequence of 30 amino acid residues in the second sequence. Similarly, for a first sequence to have “100% identity” with a second sequence means that the first sequence exactly matches the second sequence with no gaps between nucleotides or amino acids.

As used herein, unless otherwise specified, reference to a percent (%) identity refers to an evaluation of homology which is performed using: (1) a BLAST 2.0 Basic BLAST homology search using blastp for amino acid searches, blastn for nucleic acid searches, and blastX for nucleic acid searches and searches of translated amino acids in all 6 open reading frames, all with standard default parameters, wherein the query sequence is filtered for low complexity regions by default (described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. & Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402, incorporated herein by reference in its entirety); (2) a BLAST 2 alignment (using the parameters described below); (3) and/or PSI-BLAST with the standard default parameters (Position-Specific Iterated BLAST). It is noted that due to some differences in the standard parameters between BLAST 2.0 Basic BLAST and BLAST 2, two specific sequences might be recognized as having significant homology using the BLAST 2 program, whereas a search performed in BLAST 2.0 Basic BLAST using one of the sequences as the query sequence may not identify the second sequence in the top matches. In addition, PSI-BLAST provides an automated, easy-to-use version of a “profile” search, which is a sensitive way to look for sequence homologues. The program first performs a gapped BLAST database search. The PSI-BLAST program uses the information from any significant alignments returned to construct a position-specific score matrix, which replaces the query sequence for the next round of database searching. Therefore, it is to be understood that percent identity can be determined by using any one of these programs.

Two specific sequences can be aligned to one another using BLAST 2 sequence as described in Tatusova and Madden, (1999), “Blast 2 sequences—a new tool for comparing protein and nucleotide sequences”, FEMS Microbiol Lett. 174:247-250, incorporated herein by reference in its entirety. BLAST 2 sequence alignment is performed in blastp or blastn using the BLAST 2.0 algorithm to perform a Gapped BLAST search (BLAST 2.0) between the two sequences allowing for the introduction of gaps (deletions and insertions) in the resulting alignment. For purposes of clarity herein, a BLAST 2 sequence alignment is performed using the standard default parameters as follows.

For blastn, using 0 BLOSUM62 matrix:

Reward for match=1

Penalty for mismatch=−2

Open gap (5) and extension gap (2) penalties

gap x_dropoff (50) expect (10) word size (11) filter (on)

For blastp, using 0 BLOSUM62 matrix:

Open gap (11) and extension gap (1) penalties

gap x_dropoff (50) expect (10) word size (3) filter (on).

In another embodiment of the invention, an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system of the present invention includes an amino acid sequence that is sufficiently similar to a naturally occurring PUFA PKS protein or polypeptide that a nucleic acid sequence encoding the amino acid sequence is capable of hybridizing under moderate, high, or very high stringency conditions (described below) to (i.e., with) a nucleic acid molecule encoding the naturally occurring PUFA PKS protein or polypeptide (i.e., to the complement of the nucleic acid strand encoding the naturally occurring PUFA PKS protein or polypeptide). Preferably, an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system of the present invention is encoded by a nucleic acid sequence that hybridizes under moderate, high or very high stringency conditions to the complement of a nucleic acid sequence that encodes a protein comprising an amino acid sequence represented by any of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, or SEQ ID NO:32. Methods to deduce a complementary sequence are known to those skilled in the art. It should be noted that since amino acid sequencing and nucleic acid sequencing technologies are not entirely error-free, the sequences presented herein, at best, represent apparent sequences of PUFA PKS domains and proteins of the present invention.

As used herein, hybridization conditions refer to standard hybridization conditions under which nucleic acid molecules are used to identify similar nucleic acid molecules. Such standard conditions are disclosed, for example, in Sambrook et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Labs Press, 1989. Sambrook et al., ibid., is incorporated by reference herein in its entirety (see specifically, pages 9.31-9.62). In addition, formulae to calculate the appropriate hybridization and wash conditions to achieve hybridization permitting varying degrees of mismatch of nucleotides are disclosed, for example, in Meinkoth et al., 1984, Anal Biochem. 138, 267-284; Meinkoth et al., ibid., is incorporated by reference herein in its entirety.

More particularly, moderate stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 70% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 30% or less mismatch of nucleotides). High stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 80% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 20% or less mismatch of nucleotides). Very high stringency hybridization and washing conditions, as referred to herein, refer to conditions which permit isolation of nucleic acid molecules having at least about 90% nucleic acid sequence identity with the nucleic acid molecule being used to probe in the hybridization reaction (i.e., conditions permitting about 10% or less mismatch of nucleotides). As discussed above, one of skill in the art can use the formulae in Meinkoth et al., ibid. to calculate the appropriate hybridization and wash conditions to achieve these particular levels of nucleotide mismatch. Such conditions will vary, depending on whether DNA:RNA or DNA:DNA hybrids are being formed. Calculated melting temperatures for DNA:DNA hybrids are 10° C. less than for DNA:RNA hybrids. In particular embodiments, stringent hybridization conditions for DNA:DNA hybrids include hybridization at an ionic strength of 6×SSC (0.9 M Na⁺) at a temperature of between about 20° C. and about 35° C. (lower stringency), more preferably, between about 28° C. and about 40° C. (more stringent), and even more preferably, between about 35° C. and about 45° C. (even more stringent), with appropriate wash conditions. In particular embodiments, stringent hybridization conditions for DNA:RNA hybrids include hybridization at an ionic strength of 6×SSC (0.9 M Na⁺) at a temperature of between about 30° C. and about 45° C., more preferably, between about 38° C. and about 50° C., and even more preferably, between about 45° C. and about 55° C., with similarly stringent wash conditions. These values are based on calculations of a melting temperature for molecules larger than about 100 nucleotides, 0% formamide and a G+C content of about 40%. Alternatively, T_(m) can be calculated empirically as set forth in Sambrook et al., supra, pages 9.31 to 9.62. In general, the wash conditions should be as stringent as possible, and should be appropriate for the chosen hybridization conditions. For example, hybridization conditions can include a combination of salt and temperature conditions that are approximately 20-25° C. below the calculated T_(m) of a particular hybrid, and wash conditions typically include a combination of salt and temperature conditions that are approximately 12-20° C. below the calculated T_(m) of the particular hybrid. One example of hybridization conditions suitable for use with DNA:DNA hybrids includes a 2-24 hour hybridization in 6×SSC (50% formamide) at about 42° C., followed by washing steps that include one or more washes at room temperature in about 2×SSC, followed by additional washes at higher temperatures and lower ionic strength (e.g., at least one wash as about 37° C. in about 0.1×-0.5×SSC, followed by at least one wash at about 68° C. in about 0.1×-0.5×SSC).

Another embodiment of the present invention includes a recombinant nucleic acid molecule comprising a recombinant vector and a nucleic acid molecule comprising a nucleic acid sequence encoding an amino acid sequence having a biological activity of at least one domain of a PUFA PKS system as described herein. Such nucleic acid sequences are described in detail above. According to the present invention, a recombinant vector is an engineered (i.e., artificially produced) nucleic acid molecule that is used as a tool for manipulating a nucleic acid sequence of choice and for introducing such a nucleic acid sequence into a host cell. The recombinant vector is therefore suitable for use in cloning, sequencing, and/or otherwise manipulating the nucleic acid sequence of choice, such as by expressing and/or delivering the nucleic acid sequence of choice into a host cell to form a recombinant cell. Such a vector typically contains heterologous nucleic acid sequences, that is nucleic acid sequences that are not naturally found adjacent to nucleic acid sequence to be cloned or delivered, although the vector can also contain regulatory nucleic acid sequences (e.g., promoters, untranslated regions) which are naturally found adjacent to nucleic acid molecules of the present invention or which are useful for expression of the nucleic acid molecules of the present invention (discussed in detail below). The vector can be either RNA or DNA, either prokaryotic or eukaryotic, and typically is a plasmid. The vector can be maintained as an extrachromosomal element (e.g., a plasmid) or it can be integrated into the chromosome of a recombinant organism (e.g., a microbe or a plant). The entire vector can remain in place within a host cell, or under certain conditions, the plasmid DNA can be deleted, leaving behind the nucleic acid molecule of the present invention. The integrated nucleic acid molecule can be under chromosomal promoter control, under native or plasmid promoter control, or under a combination of several promoter controls. Single or multiple copies of the nucleic acid molecule can be integrated into the chromosome. A recombinant vector of the present invention can contain at least one selectable marker.

In one embodiment, a recombinant vector used in a recombinant nucleic acid molecule of the present invention is an expression vector. As used herein, the phrase “expression vector” is used to refer to a vector that is suitable for production of an encoded product (e.g., a protein of interest). In this embodiment, a nucleic acid sequence encoding the product to be produced (e.g., a PUFA PKS domain) is inserted into the recombinant vector to produce a recombinant nucleic acid molecule. The nucleic acid sequence encoding the protein to be produced is inserted into the vector in a manner that operatively links the nucleic acid sequence to regulatory sequences in the vector which enable the transcription and translation of the nucleic acid sequence within the recombinant host cell.

In another embodiment, a recombinant vector used in a recombinant nucleic acid molecule of the present invention is a targeting vector. As used herein, the phrase “targeting vector” is used to refer to a vector that is used to deliver a particular nucleic acid molecule into a recombinant host cell, wherein the nucleic acid molecule is used to delete or inactivate an endogenous gene within the host cell or microorganism (i.e., used for targeted gene disruption or knock-out technology). Such a vector may also be known in the art as a “knock-out” vector. In one aspect of this embodiment, a portion of the vector, but more typically, the nucleic acid molecule inserted into the vector (i.e., the insert), has a nucleic acid sequence that is homologous to a nucleic acid sequence of a target gene in the host cell (i.e., a gene which is targeted to be deleted or inactivated). The nucleic acid sequence of the vector insert is designed to bind to the target gene such that the target gene and the insert undergo homologous recombination, whereby the endogenous target gene is deleted, inactivated or attenuated (i.e., by at least a portion of the endogenous target gene being mutated or deleted).

Typically, a recombinant nucleic acid molecule includes at least one nucleic acid molecule of the present invention operatively linked to one or more transcription control sequences. As used herein, the phrase “recombinant molecule” or “recombinant nucleic acid molecule” primarily refers to a nucleic acid molecule or nucleic acid sequence operatively linked to a transcription control sequence, but can be used interchangeably with the phrase “nucleic acid molecule”, when such nucleic acid molecule is a recombinant molecule as discussed herein. According to the present invention, the phrase “operatively linked” refers to linking a nucleic acid molecule to a transcription control sequence in a manner such that the molecule is able to be expressed when transfected (i.e., transformed, transduced, transfected, conjugated or conduced) into a host cell. Transcription control sequences are sequences which control the initiation, elongation, or termination of transcription. Particularly important transcription control sequences are those which control transcription initiation, such as promoter, enhancer, operator and repressor sequences. Suitable transcription control sequences include any transcription control sequence that can function in a host cell or organism into which the recombinant nucleic acid molecule is to be introduced.

Recombinant nucleic acid molecules of the present invention can also contain additional regulatory sequences, such as translation regulatory sequences, origins of replication, and other regulatory sequences that are compatible with the recombinant cell. In one embodiment, a recombinant molecule of the present invention, including those which are integrated into the host cell chromosome, also contains secretory signals (i.e., signal segment nucleic acid sequences) to enable an expressed protein to be secreted from the cell that produces the protein. Suitable signal segments include a signal segment that is naturally associated with the protein to be expressed or any heterologous signal segment capable of directing the secretion of the protein according to the present invention. In another embodiment, a recombinant molecule of the present invention comprises a leader sequence to enable an expressed protein to be delivered to and inserted into the membrane of a host cell. Suitable leader sequences include a leader sequence that is naturally associated with the protein, or any heterologous leader sequence capable of directing the delivery and insertion of the protein to the membrane of a cell.

The present inventors have found that the Schizochytrium PUFA PKS Orfs A and B are closely linked in the genome and region between the Orfs has been sequenced. The Orfs are oriented in opposite directions and 4244 base pairs separate the start (ATG) codons (i.e. they are arranged as follows: 3′OrfA5′-4244 bp-5′OrfB3′). Examination of the 4244 bp intergenic region did not reveal any obvious Orfs (no significant matches were found on a BlastX search). Both Orfs A and B are highly expressed in Schizochytrium, at least during the time of oil production, implying that active promoter elements are embedded in this intergenic region. These genetic elements are believed to have utility as a bi-directional promoter sequence for transgenic applications. For example, in a preferred embodiment, one could clone this region, place any genes of interest at each end and introduce the construct into Schizochytrium (or some other host in which the promoters can be shown to function). It is predicted that the regulatory elements, under the appropriate conditions, would provide for coordinated, high level expression of the two introduced genes. The complete nucleotide sequence for the regulatory region containing Schizochytrium PUFA PKS regulatory elements (e.g., a promoter) is represented herein as SEQ ID NO:36.

In a similar manner, OrfC is highly expressed in Schizochytrium during the time of oil production and regulatory elements are expected to reside in the region upstream of its start codon. A region of genomic DNA upstream of OrfC has been cloned and sequenced and is represented herein as (SEQ ID NO:37). This sequence contains the 3886 nt immediately upstream of the OrfC start codon. Examination of this region did not reveal any obvious Orfs (i.e., no significant matches were found on a BlastX search). It is believed that regulatory elements contained in this region, under the appropriate conditions, will provide for high-level expression of a gene placed behind them. Additionally, under the appropriate conditions, the level of expression may be coordinated with genes under control of the A-B intergenic region (SEQ ID NO:36).

Therefore, in one embodiment, a recombinant nucleic acid molecule useful in the present invention, as disclosed herein, can include a PUFA PKS regulatory region contained within SEQ ID NO:36 and/or SEQ ID NO:37. Such a regulatory region can include any portion (fragment) of SEQ ID NO:36 and/or SEQ ID NO:37 that has at least basal PUFA PKS transcriptional activity.

One or more recombinant molecules of the present invention can be used to produce an encoded product (e.g., a PUFA PKS domain, protein, or system) of the present invention. In one embodiment, an encoded product is produced by expressing a nucleic acid molecule as described herein under conditions effective to produce the protein. A preferred method to produce an encoded protein is by transfecting a host cell with one or more recombinant molecules to form a recombinant cell. Suitable host cells to transfect include, but are not limited to, any bacterial, fungal (e.g., yeast), insect, plant or animal cell that can be transfected. Host cells can be either untransfected cells or cells that are already transfected with at least one other recombinant nucleic acid molecule.

According to the present invention, the term “transfection” is used to refer to any method by which an exogenous nucleic acid molecule (i.e., a recombinant nucleic acid molecule) can be inserted into a cell. The term “transformation” can be used interchangeably with the term “transfection” when such term is used to refer to the introduction of nucleic acid molecules into microbial cells, such as algae, bacteria and yeast. In microbial systems, the term “transformation” is used to describe an inherited change due to the acquisition of exogenous nucleic acids by the microorganism and is essentially synonymous with the term “transfection.” However, in animal cells, transformation has acquired a second meaning which can refer to changes in the growth properties of cells in culture after they become cancerous, for example. Therefore, to avoid confusion, the term “transfection” is preferably used with regard to the introduction of exogenous nucleic acids into animal cells, and the term “transfection” will be used herein to generally encompass transfection of animal cells, plant cells and transformation of microbial cells, to the extent that the terms pertain to the introduction of exogenous nucleic acids into a cell. Therefore, transfection techniques include, but are not limited to, transformation, particle bombardment, electroporation, microinjection, lipofection, adsorption, infection and protoplast fusion.

It will be appreciated by one skilled in the art that use of recombinant DNA technologies can improve control of expression of transfected nucleic acid molecules by manipulating, for example, the number of copies of the nucleic acid molecules within the host cell, the efficiency with which those nucleic acid molecules are transcribed, the efficiency with which the resultant transcripts are translated, and the efficiency of post-translational modifications. Additionally, the promoter sequence might be genetically engineered to improve the level of expression as compared to the native promoter. Recombinant techniques useful for controlling the expression of nucleic acid molecules include, but are not limited to, integration of the nucleic acid molecules into one or more host cell chromosomes, addition of vector stability sequences to plasmids, substitutions or modifications of transcription control signals (e.g., promoters, operators, enhancers), substitutions or modifications of translational control signals (e.g., ribosome binding sites, Shine-Dalgarno sequences), modification of nucleic acid molecules to correspond to the codon usage of the host cell, and deletion of sequences that destabilize transcripts.

General discussion above with regard to recombinant nucleic acid molecules and transfection of host cells is intended to be applied to any recombinant nucleic acid molecule discussed herein, including those encoding any amino acid sequence having a biological activity of at least one domain from a PUFA PKS, those encoding amino acid sequences from other PKS systems, and those encoding other proteins or domains.

This invention also relates to the use of a novel method to identify a microorganism that has a PUFA PKS system that is homologous in structure, domain organization and/or function to a Schizochytrium PUFA PKS system. In one embodiment, the microorganism is a non-bacterial microorganism, and preferably, the microorganism identified by this method is a eukaryotic microorganism. In addition, this invention relates to the microorganisms identified by such method and to the use of these microorganisms and the PUFA PKS systems from these microorganisms in the various applications for a PUFA PKS system (e.g., genetically modified organisms and methods of producing bioactive molecules) according to the present invention. The unique screening method described and demonstrated herein enables the rapid identification of new microbial strains containing a PUFA PKS system homologous to the Schizochytrium PUFA PKS system of the present invention. Applicants have used this method to discover and disclose herein that a Thraustochytrium microorganism contains a PUFA PKS system that is homologous to that found in Schizochytrium. This discovery is described in detail in Example 2 below.

Microbial organisms with a PUFA PKS system similar to that found in Schizochytrium, such as the Thraustochytrium microorganism discovered by the present inventors and described in Example 2, can be readily identified/isolated/screened by the following methods used separately or in any combination of these methods.

In general, the method to identify a non-bacterial microorganism that has a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system includes a first step of (a) selecting a microorganism that produces at least one PUFA; and a second step of (b) identifying a microorganism from (a) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation in the fermentation medium, as compared to production of PUFAs by said microorganism under dissolved oxygen conditions of greater than 5% of saturation, more preferably 10% of saturation, more preferably greater than 15% of saturation and more preferably greater than 20% of saturation in the fermentation medium. A microorganism that produces at least one PUFA and has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation is identified as a candidate for containing a PUFA PKS system. Subsequent to identifying a microorganism that is a strong candidate for containing a PUFA PKS system, the method can include an additional step (c) of detecting whether the organism identified in step (b) comprises a PUFA PKS system.

In one embodiment of the present invention, step (b) is performed by culturing the microorganism selected for the screening process in low oxygen/anoxic conditions and aerobic conditions, and, in addition to measuring PUFA content in the organism, the fatty acid profile is determined, as well as fat content. By comparing the results under low oxygen/anoxic conditions with the results under aerobic conditions, the method provides a strong indication of whether the test microorganism contains a PUFA PKS system of the present invention. This preferred embodiment is described in detail below.

Initially, microbial strains to be examined for the presence of a PUFA PKS system are cultured under aerobic conditions to induce production of a large number of cells (microbial biomass). As one element of the identification process, these cells are then placed under low oxygen or anoxic culture conditions (e.g., dissolved oxygen less than about 5% of saturation, more preferably less than about 2%, even more preferably less than about 1%, and most preferably dissolved oxygen of about 0% of saturation in the culture medium) and allowed to grow for approximately another 24-72 hours. In this process, the microorganisms should be cultured at a temperature greater than about 15° C., and more preferably greater than about 20° C., and even more preferably greater than about 25° C., and even more preferably greater than 30° C. The low or anoxic culture environment can be easily maintained in culture chambers capable of inducing this type of atmospheric environment in the chamber (and thus in the cultures) or by culturing the cells in a manner that induces the low oxygen environment directly in the culture flask/vessel itself.

In a preferred culturing method, the microbes can be cultured in shake flasks which, instead of normally containing a small amount of culture medium—less than about 50% of total capacity and usually less than about 25% of total capacity—to keep the medium aerated as it is shaken on a shaker table, are instead filled to greater than about 50% of their capacity, and more preferably greater than about 60%, and most preferably greater than about 75% of their capacity with culture medium. High loading of the shake flask with culture medium prevents it from mixing very well in the flask when it is placed on a shaker table, preventing oxygen diffusion into the culture. Therefore as the microbes grow, they use up the existing oxygen in the medium and naturally create a low or no oxygen environment in the shake flask.

After the culture period, the cells are harvested and analyzed for content of bioactive compounds of interest (e.g., lipids), but most particularly, for compounds containing two or more unsaturated bonds, and more preferably three or more double bonds, and even more preferably four or more double bonds. For lipids, those strains possessing such compounds at greater than about 5%, and more preferably greater than about 10%, and more preferably greater than about 15%, and even more preferably greater than about 20% of the dry weight of the microorganism are identified as predictably containing a novel PKS system of the type described above. For other bioactive compounds, such as antibiotics or compounds that are synthesized in smaller amounts, those strains possessing such compounds at greater than about 0.5%, and more preferably greater than about 0.1%, and more preferably greater than about 0.25%, and more preferably greater than about 0.5%, and more preferably greater than about 0.75%, and more preferably greater than about 1%, and more preferably greater than about 2.5%, and more preferably greater than about 5% of the dry weight of the microorganism are identified as predictably containing a novel PKS system of the type described above.

Alternatively, or in conjunction with this method, prospective microbial strains containing novel PUFA PKS systems as described herein can be identified by examining the fatty acid profile of the strain (obtained by culturing the organism or through published or other readily available sources). If the microbe contains greater than about 30%, and more preferably greater than about 40%, and more preferably greater than about 45%, and even more preferably greater than about 50% of its total fatty acids as C14:0, C16:0 and/or C16:1, while also producing at least one long chain fatty acid with three or more unsaturated bonds, and more preferably 4 or more double bonds, and more preferably 5 or more double bonds, and even more preferably 6 or more double bonds, then this microbial strain is identified as a likely candidate to possess a novel PUFA PKS system of the type described in this invention. Screening this organism under the low oxygen conditions described above, and confirming production of bioactive molecules containing two or more unsaturated bonds would suggest the existence of a novel PUFA PKS system in the organism, which could be further confirmed by analysis of the microbes' genome.

The success of this method can also be enhanced by screening eukaryotic strains that are known to contain C17:0 and or C17:1 fatty acids (in conjunction with the large percentages of C14:0, C16:0 and C16:1 fatty acids described above)—because the C17:0 and C17:1 fatty acids are potential markers for a bacterial (prokaryotic) based or influenced fatty acid production system. Another marker for identifying strains containing novel PUFA PKS systems is the production of simple fatty acid profiles by the organism. According to the present invention, a “simple fatty acid profile” is defined as 8 or fewer fatty acids being produced by the strain at levels greater than 10% of total fatty acids.

Use of any of these methods or markers (singly or preferably in combination) would enable one of skill in the art to readily identify microbial strains that are highly predicted to contain a novel PUFA PKS system of the type described in this invention.

In a preferred embodiment combining many of the methods and markers described above, a novel biorational screen (using shake flask cultures) has been developed for detecting microorganisms containing PUFA producing PKS systems. This screening system is conducted as follows:

A portion of a culture of the strain/microorganism to be tested is placed in 250 mL baffled shake flask with 50 mL culture media (aerobic treatment), and another portion of culture of the same strain is placed in a 250 mL non-baffled shake flask with 200 mL culture medium (anoxic/low oxygen treatment). Various culture media can be employed depending on the type and strain of microorganism being evaluated. Both flasks are placed on a shaker table at 200 rpm. After 48-72 hr of culture time, the cultures are harvested by centrifugation and the cells are analyzed for fatty acid methyl ester content via gas chromatography to determine the following data for each culture: (1) fatty acid profile; (2) PUFA content; and (3) fat content (approximated as amount total fatty acids/cell dry weight).

These data are then analyzed asking the following five questions (Yes/No):

Comparing the Data from the Low O₂/Anoxic Flask with the Data from the Aerobic Flask:

(1) Did the DHA (or other PUFA content) (as % FAME (fatty acid methyl esters)) stay about the same or preferably increased in the low oxygen culture compared to the aerobic culture?

(2) Is C14:0+C16:0+C16:1 greater than about 40% TFA in the anoxic culture?

(3) Are there very little (<1% as FAME) or no precursors (C18:3n−3+C18:2n−6+C18:3n−6) to the conventional oxygen dependent elongase/desaturase pathway in the anoxic culture?

(4) Did fat content (as amount total fatty acids/cell dry weight) increase in the low oxygen culture compared to the aerobic culture?

(5) Did DHA (or other PUFA content) increase as % cell dry weight in the low oxygen culture compared to the aerobic culture?

If the first three questions are answered yes, this is a good indication that the strain contains a PKS genetic system for making long chain PUFAs. The more questions that are answered yes (preferably the first three questions must be answered yes), the stronger the indication that the strain contains such a PKS genetic system. If all five questions are answered yes, then there is a very strong indication that the strain contains a PKS genetic system for making long chain PUFAs. The lack of 18:3n−3/18:2n−6/18:3n−6 would indicate that the low oxygen conditions would have turned off or inhibited the conventional pathway for PUFA synthesis. A high 14:0/16:0/16:1 fatty is an preliminary indicator of a bacterially influenced fatty acid synthesis profile (the presence of C17:0 and 17:1 is also and indicator of this) and of a simple fatty acid profile. The increased PUFA synthesis and PUFA containing fat synthesis under the low oxygen conditions is directly indicative of a PUFA PKS system, since this system does not require oxygen to make highly unsaturated fatty acids.

Finally, in the identification method of the present invention, once a strong candidate is identified, the microbe is preferably screened to detect whether or not the microbe contains a PUFA PKS system. For example, the genome of the microbe can be screened to detect the presence of one or more nucleic acid sequences that encode a domain of a PUFA PKS system as described herein. Preferably, this step of detection includes a suitable nucleic acid detection method, such as hybridization, amplification and or sequencing of one or more nucleic acid sequences in the microbe of interest. The probes and/or primers used in the detection methods can be derived from any known PUFA PKS system, including the marine bacteria PUFA PKS systems described in U.S. Pat. No. 6,140,486, or the Thraustochytrid PUFA PKS systems described in U.S. application Ser. No. 09/231,899 and herein. Once novel PUFA PKS systems are identified, the genetic material from these systems can also be used to detect additional novel PUFA PKS systems. Methods of hybridization, amplification and sequencing of nucleic acids for the purpose of identification and detection of a sequence are well known in the art. Using these detection methods, sequence homology and domain structure (e.g., the presence, number and/or arrangement of various PUFA PKS functional domains) can be evaluated and compared to the known PUFA PKS systems described herein.

In some embodiments, a PUFA PKS system can be identified using biological assays. For example, in U.S. application Ser. No. 09/231,899, Example 7, the results of a key experiment using a well-known inhibitor of some types of fatty acid synthesis systems, i.e., thiolactomycin, is described. The inventors showed that the synthesis of PUFAs in whole cells of Schizochytrium could be specifically blocked without blocking the synthesis of short chain saturated fatty acids. The significance of this result is as follows: the inventors knew from analysis of cDNA sequences from Schizochytrium that a Type I fatty acid synthase system is present in Schizochytrium. It was known that thiolactomycin does not inhibit Type I FAS systems, and this is consistent with the inventors' data—i.e., production of the saturated fatty acids (primarily C14:0 and C16:0 in Schizochytrium) was not inhibited by the thiolactomycin treatment. There are no indications in the literature or in the inventors' own data that thiolactomycin has any inhibitory effect on the elongation of C14:0 or C16:0 fatty acids or their desaturation (i.e. the conversion of short chain saturated fatty acids to PUFAs by the classical pathway). Therefore, the fact that the PUFA production in Schizochytrium was blocked by thiolactomycin strongly indicates that the classical PUFA synthesis pathway does not produce the PUFAs in Schizochytrium, but rather that a different pathway of synthesis is involved. Further, it had previously been determined that the Shewanella PUFA PKS system is inhibited by thiolactomycin (note that the PUFA PKS system of the present invention has elements of both Type I and Type II systems), and it was known that thiolactomycin is an inhibitor of Type II FAS systems (such as that found in E. coli). Therefore, this experiment indicated that Schizochytrium produced PUFAs as a result of a pathway not involving the Type I FAS. A similar rationale and detection step could be used to detect a PUFA PKS system in a microbe identified using the novel screening method disclosed herein.

In addition, Example 3 shows additional biochemical data which provides evidence that PUFAs in Schizochytrium are not produced by the classical pathway (i.e., precursor product kinetics between C16:0 and DHA are not observed in whole cells and, in vitro PUFA synthesis can be separated from the membrane fraction—all of the fatty acid desaturases of the classical PUFA synthesis pathway, with the exception of the delta 9 desaturase which inserts the first double bond of the series, are associated with cellular membranes). This type of biochemical data could be used to detect PUFA PKS activity in microbe identified by the novel screening method described above.

Preferred microbial strains to screen using the screening/identification method of the present invention are chosen from the group consisting of: bacteria, algae, fungi, protozoa or protists, but most preferably from the eukaryotic microbes consisting of algae, fungi, protozoa and protists. These microbes are preferably capable of growth and production of the bioactive compounds containing two or more unsaturated bonds at temperatures greater than about 15° C., more preferably greater than about 20° C., even more preferably greater than about 25° C. and most preferably greater than about 30° C.

In some embodiments of this method of the present invention, novel bacterial PUFA PKS systems can be identified in bacteria that produce PUFAs at temperatures exceeding about 20° C., preferably exceeding about 25° C. and even more preferably exceeding about 30° C. As described previously herein, the marine bacteria, Shewanella and Vibrio marinus, described in U.S. Pat. No. 6,140,486, do not produce PUFAs at higher temperatures, which limits the usefulness of PUFA PKS systems derived from these bacteria, particularly in plant applications under field conditions. Therefore, in one embodiment, the screening method of the present invention can be used to identify bacteria that have a PUFA PKS system which are capable of growth and PUFA production at higher temperatures (e.g., above about 20, 25, or 30° C.). In this embodiment, inhibitors of eukaryotic growth such as nystatin (antifungal) or cycloheximide (inhibitor of eukaryotic protein synthesis) can be added to agar plates used to culture/select initial strains from water samples/soil samples collected from the types of habitats/niches described below. This process would help select for enrichment of bacterial strains without (or minimal) contamination of eukaryotic strains. This selection process, in combination with culturing the plates at elevated temperatures (e.g. 30° C.), and then selecting strains that produce at least one PUFA would initially identify candidate bacterial strains with a PUFA PKS system that is operative at elevated temperatures (as opposed to those bacterial strains in the prior art which only exhibit PUFA production at temperatures less than about 20° C. and more preferably below about 5° C.).

Locations for collection of the preferred types of microbes for screening for a PUFA PKS system according to the present invention include any of the following: low oxygen environments (or locations near these types of low oxygen environments including in the guts of animals including invertebrates that consume microbes or microbe-containing foods (including types of filter feeding organisms), low or non-oxygen containing aquatic habitats (including freshwater, saline and marine), and especially at- or near-low oxygen environments (regions) in the oceans. The microbial strains would preferably not be obligate anaerobes but be adapted to live in both aerobic and low or anoxic environments. Soil environments containing both aerobic and low oxygen or anoxic environments would also excellent environments to find these organisms in and especially in these types of soil in aquatic habitats or temporary aquatic habitats.

A particularly preferred microbial strain would be a strain (selected from the group consisting of algae, fungi (including yeast), protozoa or protists) that, during a portion of its life cycle, is capable of consuming whole bacterial cells (bacterivory) by mechanisms such as phagocytosis, phagotrophic or endocytic capability and/or has a stage of its life cycle in which it exists as an amoeboid stage or naked protoplast. This method of nutrition would greatly increase the potential for transfer of a bacterial PKS system into a eukaryotic cell if a mistake occurred and the bacterial cell (or its DNA) did not get digested and instead are functionally incorporated into the eukaryotic cell.

Strains of microbes (other than the members of the Thraustochytrids) capable of bacterivory (especially by phagocytosis or endocytosis) can be found in the following microbial classes (including but not limited to example genera):

In the algae and algae-like microbes (including stramenopiles): of the class Euglenophyceae (for example genera Euglena, and Peranema), the class Chrysophyceae (for example the genus Ochromonas), the class Dinobryaceae (for example the genera Dinobryon, Platychrysis, and Chrysochromulina), the Dinophyceae (including the genera Crypthecodinium, Gymnodinium, Peridinium, Ceratium, Gyrodinium, and Oxyrrhis), the class Cryptophyceae (for example the genera Cryptomonas, and Rhodomonas), the class Xanthophyceae (for example the genus Olisthodiscus) (and including forms of algae in which an amoeboid stage occurs as in the flagellates Rhizochloridaceae, and zoospores/gametes of Aphanochaete pascheri, Bumilleria stigeoclonium and Vaucheria geminata), the class Eustigmatophyceae, and the class Prymnesiopyceae (including the genera Prymnesium and Diacronema).

In the Stramenopiles including the: Proteromonads, Opalines, Developayella, Diplophorys, Larbrinthulids, Thraustochytrids, Bicosecids, Oomycetes, Hypochytridiomycetes, Commation, Reticulosphaera, Pelagomonas, Pelapococcus, Ollicola, Aureococcus, Parmales, Raphidiophytes, Synurids, Rhizochromulinaales, Pedinellales, Dictyochales, Chrysomeridales, Sarcinochrysidales, Hydrurales, Hibberdiales, and Chromulinales.

In the Fungi: Class Myxomycetes (form myxamoebae)—slime molds, class Acrasieae including the orders Acrasiceae (for example the genus Sappinia), class Guttulinaceae (for example the genera Guttulinopsis, and Guttulina), class Dictysteliaceae (for example the genera Acrasis, Dictyostelium, Polysphondylium, and Coenonia), and class Phycomyceae including the orders Chytridiales, Ancylistales, Blastocladiales, Monoblepharidales, Saprolegniales, Peronosporales, Mucorales, and Entomophthorales.

In the Protozoa: Protozoa strains with life stages capable of bacterivory (including by phageocytosis) can be selected from the types classified as ciliates, flagellates or amoebae. Protozoan ciliates include the groups: Chonotrichs, Colpodids, Cyrtophores, Haptorids, Karyorelicts, Oligohymenophora, Polyhymenophora (spirotrichs), Prostomes and Suctoria. Protozoan flagellates include the Biosoecids, Bodonids, Cercomonads, Chrysophytes (for example the genera Anthophysa, Chrysamoemba, Chrysosphaerella, Dendromonas, Dinobryon, Mallomonas, Ochromonas, Paraphysomonas, Poterioochromonas, Spumella, Syncrypta, Synura, and Uroglena), Collar flagellates, Cryptophytes (for example the genera Chilomonas, Cryptomonas, Cyanomonas, and Goniomonas), Dinoflagellates, Diplomonads, Euglenoids, Heterolobosea, Pedinellids, Pelobionts, Phalansteriids, Pseudodendromonads, Spongomonads and Volvocales (and other flagellates including the unassigned flagellate genera of Artodiscus, Clautriavia, Helkesimastix, Kathablepharis and Multicilia). Amoeboid protozoans include the groups: Actinophryids, Centrohelids, Desmothoricids, Diplophryids, Eumamoebae, Heterolobosea, Leptomyxids, Nucleariid filose amoebae, Pelebionts, Testate amoebae and Vampyrellids (and including the unassigned amoebid genera Gymnophrys, Biomyxa, Microcometes, Reticulomyxa, Belonocystis, Elaeorhanis, Allelogromia, Gromia or Lieberkuhnia). The protozoan orders include the following: Percolomonadeae, Heterolobosea, Lyromonadea, Pseudociliata, Trichomonadea, Hypermastigea, Heteromiteae, Telonemea, Cyathobodonea, Ebridea, Pyytomyxea, Opalinea, Kinetomonadea, Hemimastigea, Protostelea, Myxagastrea, Dictyostelea, Choanomonadea, Apicomonadea, Eogregarinea, Neogregarinea, Coelotrolphea, Eucoccidea, Haemosporea, Piroplasmea, Spirotrichea, Prostomatea, Litostomatea, Phyllopharyngea, Nassophorea, Oligohymenophorea, Colpodea, Karyorelicta, Nucleohelea, Centrohelea, Acantharea, Sticholonchea, Polycystinea, Phaeodarea, Lobosea, Filosea, Athalamea, Monothalamea, Polythalamea, Xenophyophorea, Schizocladea, Holosea, Entamoebea, Myxosporea, Actinomyxea, Halosporea, Paramyxea, Rhombozoa and Orthonectea.

A preferred embodiment of the present invention includes strains of the microorganisms listed above that have been collected from one of the preferred habitats listed above.

One embodiment of the present invention relates to any microorganisms identified using the novel PUFA PKS screening method described above, to the PUFA PKS genes and proteins encoded thereby, and to the use of such microorganisms and/or PUFA PKS genes and proteins (including homologues and fragments thereof) in any of the methods described herein. In particular, the present invention encompasses organisms identified by the screening method of the present invention which are then genetically modified to regulate the production of bioactive molecules by said PUFA PKS system.

Yet another embodiment of the present invention relates to an isolated nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain or biologically active fragment thereof of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism. As discussed above, the present inventors have successfully used the method to identify a non-bacterial microorganism that has a PUFA PKS system to identify additional members of the order Thraustochytriales which contain a PUFA PKS system. The identification of three such microorganisms is described in Example 2. Specifically, the present inventors have used the screening method of the present invention to identify Thraustochytrium sp. 23B (ATCC 20892) as being highly predicted to contain a PUFA PKS system, followed by detection of sequences in the Thraustochytrium sp. 23B genome that hybridize to the Schizochytrium PUFA PKS genes disclosed herein. Schizochytrium limacium (IFO 32693) and Ulkenia (BP-5601) have also been identified as good candidates for containing PUFA PKS systems. Based on these data and on the similarities among members of the order Thraustochytriales, it is believed that many other Thraustochytriales PUFA PKS systems can now be readily identified using the methods and tools provided by the present invention. Therefore, Thraustochytriales PUFA PKS systems and portions and/or homologues thereof (e.g., proteins, domains and fragments thereof), genetically modified organisms comprising such systems and portions and/or homologues thereof, and methods of using such microorganisms and PUFA PKS systems, are encompassed by the present invention.

Developments have resulted in revision of the taxonomy of the Thraustochytrids. Taxonomic theorists place Thraustochytrids with the algae or algae-like protists. However, because of taxonomic uncertainty, it would be best for the purposes of the present invention to consider the strains described in the present invention as Thraustochytrids (Order: Thraustochytriales; Family: Thraustochytriaceae; Genus: Thraustochytrium, Schizochytrium, Labyrinthuloides, or Japonochytrium). For the present invention, members of the labrinthulids are considered to be included in the Thraustochytrids. Taxonomic changes are summarized below. Strains of certain unicellular microorganisms disclosed herein are members of the order Thraustochytriales. Thraustochytrids are marine eukaryotes with a evolving taxonomic history. Problems with the taxonomic placement of the Thraustochytrids have been reviewed by Moss (1986), Bahnweb and Jackle (1986) and Chamberlain and Moss (1988). According to the present invention, the phrases “Thraustochytrid”, “Thraustochytriales microorganism” and “microorganism of the order Thraustochytriales” can be used interchangeably.

For convenience purposes, the Thraustochytrids were first placed by taxonomists with other colorless zoosporic eukaryotes in the Phycomycetes (algae-like fungi). The name Phycomycetes, however, was eventually dropped from taxonomic status, and the Thraustochytrids were retained in the Oomycetes (the biflagellate zoosporic fungi). It was initially assumed that the Oomycetes were related to the heterokont algae, and eventually a wide range of ultrastructural and biochemical studies, summarized by Barr (Barr, 1981, Biosystems 14:359-370) supported this assumption. The Oomycetes were in fact accepted by Leedale (Leedale, 1974, Taxon 23:261-270) and other phycologists as part of the heterokont algae. However, as a matter of convenience resulting from their heterotrophic nature, the Oomycetes and Thraustochytrids have been largely studied by mycologists (scientists who study fungi) rather than phycologists (scientists who study algae).

From another taxonomic perspective, evolutionary biologists have developed two general schools of thought as to how eukaryotes evolved. One theory proposes an exogenous origin of membrane-bound organelles through a series of endosymbioses (Margulis, 1970, Origin of Eukaryotic Cells. Yale University Press, New Haven); e.g., mitochondria were derived from bacterial endosymbionts, chloroplasts from cyanophytes, and flagella from spirochaetes. The other theory suggests a gradual evolution of the membrane-bound organelles from the non-membrane-bounded systems of the prokaryote ancestor via an autogenous process (Cavalier-Smith, 1975, Nature (Lond.) 256:462-468). Both groups of evolutionary biologists however, have removed the Oomycetes and Thraustochytrids from the fungi and place them either with the chromophyte algae in the kingdom Chromophyta (Cavalier-Smith, 1981, BioSystems 14:461-481) (this kingdom has been more recently expanded to include other protists and members of this kingdom are now called Stramenopiles) or with all algae in the kingdom Protoctista (Margulis and Sagen, 1985, Biosystems 18:141-147).

With the development of electron microscopy, studies on the ultrastructure of the zoospores of two genera of Thraustochytrids, Thraustochytrium and Schizochytrium, (Perkins, 1976, pp. 279-312 in “Recent Advances in Aquatic Mycology” (ed. E. B. G. Jones), John Wiley & Sons, New York; Kazama, 1980, Can. J. Bot. 58:2434-2446; Barr, 1981, Biosystems 14:359-370) have provided good evidence that the Thraustochytriaceae are only distantly related to the Oomycetes. Additionally, genetic data representing a correspondence analysis (a form of multivariate statistics) of 5 S ribosomal RNA sequences indicate that Thraustochytriales are clearly a unique group of eukaryotes, completely separate from the fungi, and most closely related to the red and brown algae, and to members of the Oomycetes (Mannella, et al., 1987, Mol. Evol. 24:228-235). Most taxonomists have agreed to remove the Thraustochytrids from the Oomycetes (Bartnicki-Garcia, 1987, pp. 389-403 in “Evolutionary Biology of the Fungi” (eds. Rayner, A. D. M., Brasier, C. M. & Moore, D.), Cambridge University Press, Cambridge).

In summary, employing the taxonomic system of Cavalier-Smith (Cavalier-Smith, 1981, BioSystems 14:461-481, 1983; Cavalier-Smith, 1993, Microbiol Rev. 57:953-994), the Thraustochytrids are classified with the chromophyte algae in the kingdom Chromophyta (Stramenopiles). This taxonomic placement has been more recently reaffirmed by Cavalier-Smith et al. using the 18s rRNA signatures of the Heterokonta to demonstrate that Thraustochytrids are chromists not Fungi (Cavalier-Smith et al., 1994, Phil. Tran. Roy. Soc. London Series BioSciences 346:387-397). This places them in a completely different kingdom from the fungi, which are all placed in the kingdom Eufungi. The taxonomic placement of the Thraustochytrids is therefore summarized below:

-   Kingdom: Chromophyta (Stramenopiles) -   Phylum: Heterokonta -   Order: Thraustochytriales -   Family: Thraustochytriaceae     Genus: Thraustochytrium, Schizochytrium, Labyrinthuloides, or     Japonochytrium

Some early taxonomists separated a few original members of the genus Thraustochytrium (those with an amoeboid life stage) into a separate genus called Ulkenia. However it is now known that most, if not all, Thraustochytrids (including Thraustochytrium and Schizochytrium), exhibit amoeboid stages and as such, Ulkenia is not considered by some to be a valid genus. As used herein, the genus Thraustochytrium will include Ulkenia.

Despite the uncertainty of taxonomic placement within higher classifications of Phylum and Kingdom, the Thraustochytrids remain a distinctive and characteristic grouping whose members remain classifiable within the order Thraustochytriales.

Polyunsaturated fatty acids (PUFAs) are essential membrane components in higher eukaryotes and the precursors of many lipid-derived signaling molecules. The PUFA PKS system of the present invention uses pathways for PUFA synthesis that do not require desaturation and elongation of saturated fatty acids. The pathways catalyzed by PUFA PKSs that are distinct from previously recognized PKSs in both structure and mechanism. Generation of cis double bonds is suggested to involve position-specific isomerases; these enzymes are believed to be useful in the production of new families of antibiotics.

To produce significantly high yields of various bioactive molecules using the PUFA PKS system of the present invention, an organism, preferably a microorganism or a plant, can be genetically modified to affect the activity of a PUFA PKS system. In one aspect, such an organism can endogenously contain and express a PUFA PKS system, and the genetic modification can be a genetic modification of one or more of the functional domains of the endogenous PUFA PKS system, whereby the modification has some effect on the activity of the PUFA PKS system. In another aspect, such an organism can endogenously contain and express a PUFA PKS system, and the genetic modification can be an introduction of at least one exogenous nucleic acid sequence (e.g., a recombinant nucleic acid molecule), wherein the exogenous nucleic acid sequence encodes at least one biologically active domain or protein from a second PKS system and/or a protein that affects the activity of said PUFA PKS system (e.g., a phosphopantetheinyl transferases (PPTase), discussed below). In yet another aspect, the organism does not necessarily endogenously (naturally) contain a PUFA PKS system, but is genetically modified to introduce at least one recombinant nucleic acid molecule encoding an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system. In this aspect, PUFA PKS activity is affected by introducing or increasing PUFA PKS activity in the organism. Various embodiments associated with each of these aspects will be discussed in greater detail below.

Therefore, according to the present invention, one embodiment relates to a genetically modified microorganism, wherein the microorganism expresses a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The at least one domain of the PUFA PKS system is encoded by a nucleic acid sequence chosen from: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by a screening method of the present invention; (c) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and, (d) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. The genetic modification affects the activity of the PKS system in the organism. The screening process referenced in part (b) has been described in detail above and includes the steps of: (a) selecting a microorganism that produces at least one PUFA; and, (b) identifying a microorganism from (a) that has an ability to produce increased PUFAs under dissolved oxygen conditions of less than about 5% of saturation in the fermentation medium, as compared to production of PUFAs by the microorganism under dissolved oxygen conditions of greater than about 5% of saturation, and preferably about 10%, and more preferably about 15%, and more preferably about 20% of saturation in the fermentation medium. The genetically modified microorganism can include anyone or more of the above-identified nucleic acid sequences, and/or any of the other homologues of any of the Schizochytrium PUFA PKS ORFs or domains as described in detail above.

As used herein, a genetically modified microorganism can include a genetically modified bacterium, protist, microalgae, fungus, or other microbe, and particularly, any of the genera of the order Thraustochytriales (e.g., a Thraustochytrid) described herein (e.g., Schizochytrium, Thraustochytrium, Japonochytrium, Labyrinthuloides). Such a genetically modified microorganism has a genome which is modified (i.e., mutated or changed) from its normal (i.e., wild-type or naturally occurring) form such that the desired result is achieved (i.e., increased or modified PUFA PKS activity and/or production of a desired product using the PKS system). Genetic modification of a microorganism can be accomplished using classical strain development and/or molecular genetic techniques. Such techniques known in the art and are generally disclosed for microorganisms, for example, in Sambrook et al., 1989, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Labs Press. The reference Sambrook et al., ibid., is incorporated by reference herein in its entirety. A genetically modified microorganism can include a microorganism in which nucleic acid molecules have been inserted, deleted or modified (i.e., mutated; e.g., by insertion, deletion, substitution, and/or inversion of nucleotides), in such a manner that such modifications provide the desired effect within the microorganism.

Preferred microorganism host cells to modify according to the present invention include, but are not limited to, any bacteria, protist, microalga, fungus, or protozoa. In one aspect, preferred microorganisms to genetically modify include, but are not limited to, any microorganism of the order Thraustochytriales. Particularly preferred host cells for use in the present invention could include microorganisms from a genus including, but not limited to: Thraustochytrium, Labyrinthuloides, Japonochytrium, and Schizochytrium. Preferred species within these genera include, but are not limited to: any Schizochytrium species, including Schizochytrium aggregatum, Schizochytrium limacinum, Schizochytrium minutum; any Thraustochytrium species (including former Ulkenia species such as U. visurgensis, U. amoeboida, U. sarkariana, U. profunda, U. radiata, U. minuta and Ulkenia sp. BP-5601), and including Thraustochytrium striatum, Thraustochytrium aureum, Thraustochytrium roseum; and any Japonochytrium species. Particularly preferred strains of Thraustochytriales include, but are not limited to: Schizochytrium sp. (S31) (ATCC 20888); Schizochytrium sp. (S8) (ATCC 20889); Schizochytrium sp. (LC-RM) (ATCC 18915); Schizochytrium sp. (SR21); Schizochytrium aggregatum (Goldstein et Belsky) (ATCC 28209); Schizochytrium limacinum (Honda et Yokochi)(IFO 32693); Thraustochytrium sp. (23B) (ATCC 20891); Thraustochytrium striatum (Schneider) (ATCC 24473); Thraustochytrium aureum (Goldstein) (ATCC 34304); Thraustochytrium roseum (Goldstein) (ATCC 28210); and Japonochytrium sp. (L1) (ATCC 28207). Other examples of suitable host microorganisms for genetic modification include, but are not limited to, yeast including Saccharomyces cerevisiae, Saccharomyces carlsbergensis, or other yeast such as Candida, Kluyveromyces, or other fungi, for example, filamentous fungi such as Aspergillus, Neurospora, Penicillium, etc. Bacterial cells also may be used as hosts. This includes Escherichia coli, which can be useful in fermentation processes. Alternatively, a host such as a Lactobacillus species or Bacillus species can be used as a host.

Another embodiment of the present invention relates to a genetically modified plant, wherein the plant has been genetically modified to recombinantly express a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. The domain is encoded by a nucleic acid sequence chosen from: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by the screening and selection method described herein (see brief summary of method in discussion of genetically modified microorganism above); (c) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof; (d) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof, (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and/or (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. The genetically modified plant can include any one or more of the above-identified nucleic acid sequences, and/or any of the other homologues of any of the Schizochytrium PUFA PKS ORFs or domains as described in detail above.

As used herein, a genetically modified plant can include any genetically modified plant including higher plants and particularly, any consumable plants or plants useful for producing a desired bioactive molecule of the present invention. Such a genetically modified plant has a genome which is modified (i.e., mutated or changed) from its normal (i.e., wild-type or naturally occurring) form such that the desired result is achieved (i.e., increased or modified PUFA PKS activity and/or production of a desired product using the PKS system). Genetic modification of a plant can be accomplished using classical strain development and/or molecular genetic techniques. Methods for producing a transgenic plant, wherein a recombinant nucleic acid molecule encoding a desired amino acid sequence is incorporated into the genome of the plant, are known in the art. A preferred plant to genetically modify according to the present invention is preferably a plant suitable for consumption by animals, including humans.

Preferred plants to genetically modify according to the present invention (i.e., plant host cells) include, but are not limited to any higher plants, and particularly consumable plants, including crop plants and especially plants used for their oils. Such plants can include, for example: canola, soybeans, rapeseed, linseed, corn, safflowers, sunflowers and tobacco. Other preferred plants include those plants that are known to produce compounds used as pharmaceutical agents, flavoring agents, neutraceutical agents, functional food ingredients or cosmetically active agents or plants that are genetically engineered to produce these compounds/agents.

According to the present invention, a genetically modified microorganism or plant includes a microorganism or plant that has been modified using recombinant technology. As used herein, genetic modifications which result in a decrease in gene expression, in the function of the gene, or in the function of the gene product (i.e., the protein encoded by the gene) can be referred to as inactivation (complete or partial), deletion, interruption, blockage or down-regulation of a gene. For example, a genetic modification in a gene which results in a decrease in the function of the protein encoded by such gene, can be the result of a complete deletion of the gene (i.e., the gene does not exist, and therefore the protein does not exist), a mutation in the gene which results in incomplete or no translation of the protein (e.g., the protein is not expressed), or a mutation in the gene which decreases or abolishes the natural function of the protein (e.g., a protein is expressed which has decreased or no enzymatic activity or action). Genetic modifications that result in an increase in gene expression or function can be referred to as amplification, overproduction, overexpression, activation, enhancement, addition, or up-regulation of a gene.

The genetic modification of a microorganism or plant according to the present invention preferably affects the activity of the PKS system expressed by the plant, whether the PKS system is endogenous and genetically modified, endogenous with the introduction of recombinant nucleic acid molecules into the organism, or provided completely by recombinant technology. According to the present invention, to “affect the activity of a PKS system” includes any genetic modification that causes any detectable or measurable change or modification in the PKS system expressed by the organism as compared to in the absence of the genetic modification. A detectable change or modification in the PKS system can include, but is not limited to: the introduction of PKS system activity into an organism such that the organism now has measurable/detectable PKS system activity (i.e., the organism did not contain a PKS system prior to the genetic modification), the introduction into the organism of a functional domain from a different PKS system than a PKS system endogenously expressed by the organism such that the PKS system activity is modified (e.g., a bacterial PUFA PKS domain or a type I PKS domain is introduced into an organism that endogenously expresses a non-bacterial PUFA PKS system), a change in the amount of a bioactive molecule produced by the PKS system (e.g., the system produces more (increased amount) or less (decreased amount) of a given product as compared to in the absence of the genetic modification), a change in the type of a bioactive molecule produced by the PKS system (e.g., the system produces a new or different product, or a variant of a product that is naturally produced by the system), and/or a change in the ratio of multiple bioactive molecules produced by the PKS system (e.g., the system produces a different ratio of one PUFA to another PUFA, produces a completely different lipid profile as compared to in the absence of the genetic modification, or places various PUFAs in different positions in a triacylglycerol as compared to the natural configuration). Such a genetic modification includes any type of genetic modification and specifically includes modifications made by recombinant technology and by classical mutagenesis.

It should be noted that reference to increasing the activity of a functional domain or protein in a PUFA PKS system refers to any genetic modification in the organism containing the domain or protein (or into which the domain or protein is to be introduced) which results in increased functionality of the domain or protein system and can include higher activity of the domain or protein (e.g., specific activity or in vivo enzymatic activity), reduced inhibition or degradation of the domain or protein system, and overexpression of the domain or protein. For example, gene copy number can be increased, expression levels can be increased by use of a promoter that gives higher levels of expression than that of the native promoter, or a gene can be altered by genetic engineering or classical mutagenesis to increase the activity of the domain or protein encoded by the gene.

Similarly, reference to decreasing the activity of a functional domain or protein in a PUFA PKS system refers to any genetic modification in the organism containing such domain or protein (or into which the domain or protein is to be introduced) which results in decreased functionality of the domain or protein and includes decreased activity of the domain or protein, increased inhibition or degradation of the domain or protein and a reduction or elimination of expression of the domain or protein. For example, the action of domain or protein of the present invention can be decreased by blocking or reducing the production of the domain or protein, “knocking out” the gene or portion thereof encoding the domain or protein, reducing domain or protein activity, or inhibiting the activity of the domain or protein. Blocking or reducing the production of an domain or protein can include placing the gene encoding the domain or protein under the control of a promoter that requires the presence of an inducing compound in the growth medium. By establishing conditions such that the inducer becomes depleted from the medium, the expression of the gene encoding the domain or protein (and therefore, of protein synthesis) could be turned off. Blocking or reducing the activity of domain or protein could also include using an excision technology approach similar to that described in U.S. Pat. No. 4,743,546, incorporated herein by reference. To use this approach, the gene encoding the protein of interest is cloned between specific genetic sequences that allow specific, controlled excision of the gene from the genome. Excision could be prompted by, for example, a shift in the cultivation temperature of the culture, as in U.S. Pat. No. 4,743,546, or by some other physical or nutritional signal.

In one embodiment of the present invention, a genetic modification includes a modification of a nucleic acid sequence encoding an amino acid sequence that has a biological activity of at least one domain of a non-bacterial PUFA PKS system as described herein. Such a modification can be to an amino acid sequence within an endogenously (naturally) expressed non-bacterial PUFA PKS system, whereby a microorganism that naturally contains such a system is genetically modified by, for example, classical mutagenesis and selection techniques and/or molecular genetic techniques, include genetic engineering techniques. Genetic engineering techniques can include, for example, using a targeting recombinant vector to delete a portion of an endogenous gene, or to replace a portion of an endogenous gene with a heterologous sequence. Examples of heterologous sequences that could be introduced into a host genome include sequences encoding at least one functional domain from another PKS system, such as a different non-bacterial PUFA PKS system, a bacterial PUFA PKS system, a type I PKS system, a type II PKS system, or a modular PKS system. Other heterologous sequences to introduce into the genome of a host includes a sequence encoding a protein or functional domain that is not a domain of a PKS system, but which will affect the activity of the endogenous PKS system. For example, one could introduce into the host genome a nucleic acid molecule encoding a phosphopantetheinyl transferase (discussed below). Specific modifications that could be made to an endogenous PUFA PKS system are discussed in detail below.

In another aspect of this embodiment of the invention, the genetic modification can include: (1) the introduction of a recombinant nucleic acid molecule encoding an amino acid sequence having a biological activity of at least one domain of a non-bacterial PUFA PKS system; and/or (2) the introduction of a recombinant nucleic acid molecule encoding a protein or functional domain that affects the activity of a PUFA PKS system, into a host. The host can include: (1) a host cell that does not express any PKS system, wherein all functional domains of a PKS system are introduced into the host cell, and wherein at least one functional domain is from a non-bacterial PUFA PKS system; (2) a host cell that expresses a PKS system (endogenous or recombinant) having at least one functional domain of a non-bacterial PUFA PKS system, wherein the introduced recombinant nucleic acid molecule can encode at least one additional non-bacterial PUFA PKS domain function or another protein or domain that affects the activity of the host PKS system; and (3) a host cell that expresses a PKS system (endogenous or recombinant) which does not necessarily include a domain function from a non-bacterial PUFA PKS, and wherein the introduced recombinant nucleic acid molecule includes a nucleic acid sequence encoding at least one functional domain of a non-bacterial PUFA PKS system. In other words, the present invention intends to encompass any genetically modified organism (e.g., microorganism or plant), wherein the organism comprises at least one non-bacterial PUFA PKS domain function (either endogenously or by recombinant modification), and wherein the genetic modification has a measurable effect on the non-bacterial PUFA PKS domain function or on the PKS system when the organism comprises a functional PKS system.

Therefore, using the non-bacterial PUFA PKS systems of the present invention, which, for example, makes use of genes from Thraustochytrid PUFA PKS systems, gene mixing can be used to extend the range of PUFA products to include EPA, DHA, ARA, GLA, SDA and others, as well as to produce a wide variety of bioactive molecules, including antibiotics, other pharmaceutical compounds, and other desirable products. The method to obtain these bioactive molecules includes not only the mixing of genes from various organisms but also various methods of genetically modifying the non-bacterial PUFA PKS genes disclosed herein. Knowledge of the genetic basis and domain structure of the non-bacterial PUFA PKS system of the present invention provides a basis for designing novel genetically modified organisms which produce a variety of bioactive molecules. Although mixing and modification of any PKS domains and related genes are contemplated by the present inventors, by way of example, various possible manipulations of the PUFA-PKS system are discussed below with regard to genetic modification and bioactive molecule production.

For example, in one embodiment, non-bacterial PUFA-PKS system products, such as those produced by Thraustochytrids, are altered by modifying the CLF (chain length factor) domain. This domain is characteristic of Type II (dissociated enzymes) PKS systems. Its amino acid sequence shows homology to KS (keto synthase pairs) domains, but it lacks the active site cysteine. CLF may function to determine the number of elongation cycles, and hence the chain length, of the end product. In this embodiment of the invention, using the current state of knowledge of FAS and PKS synthesis, a rational strategy for production of ARA by directed modification of the non-bacterial PUFA-PKS system is provided. There is controversy in the literature concerning the function of the CLF in PKS systems (C. Bisang et al., Nature 401, 502 (1999)) and it is realized that other domains may be involved in determination of the chain length of the end product. However, it is significant that Schizochytrium produces both DHA (C22:6, ω-3) and DPA (C22:5, ω-6). In the PUFA-PKS system the cis double bonds are introduced during synthesis of the growing carbon chain. Since placement of the ω-3 and ω-6 double bonds occurs early in the synthesis of the molecules, one would not expect that they would affect subsequent end-product chain length determination. Thus, without being bound by theory, the present inventors believe that introduction of a factor (e.g. CLF) that directs synthesis of C20 units (instead of C22 units) into the Schizochytrium PUFA-PKS system will result in the production of EPA (C20:5, ω-3) and ARA (C20:4, ω-6). For example, in heterologous systems, one could exploit the CLF by directly substituting a CLF from an EPA producing system (such as one from Photobacterium) into the Schizochytrium gene set. The fatty acids of the resulting transformants can then be analyzed for alterations in profiles to identify the transformants producing EPA and/or ARA.

In addition to dependence on development of a heterologous system (recombinant system, such as could be introduced into plants), the CLF concept can be exploited in Schizochytrium (i.e., by modification of a Schizochytrium genome). Transformation and homologous recombination has been demonstrated in Schizochytrium. One can exploit this by constructing a clone with the CLF of OrfB replaced with a CLF from a C20 PUFA-PKS system. A marker gene will be inserted downstream of the coding region. One can then transform the wild type cells, select for the marker phenotype and then screen for those that had incorporated the new CLF. Again, one would analyze these for any effects on fatty acid profiles to identify transformants producing EPA and/or ARA. If some factor other than those associated with the CLF are found to influence the chain length of the end product, a similar strategy could be employed to alter those factors.

Another preferred embodiment involving alteration of the PUFA-PKS products involves modification or substitution of the β-hydroxy acyl-ACP dehydrase/keto synthase pairs. During cis-vaccenic acid (C18:1, Δ11) synthesis in E. coli, creation of the cis double bond is believed to depend on a specific DH enzyme, β-hydroxy acyl-ACP dehydrase, the product of the FabA gene. This enzyme removes HOH from a β-keto acyl-ACP and leaves a trans double bond in the carbon chain. A subset of DH's, FabA-like, possess cis-trans isomerase activity (Heath et al., 1996, supra). A novel aspect of bacterial and non-bacterial PUFA-PKS systems is the presence of two FabA-like DH domains. Without being bound by theory, the present inventors believe that one or both of these DH domains will possess cis-trans isomerase activity (manipulation of the DH domains is discussed in greater detail below).

Another aspect of the unsaturated fatty acid synthesis in E. coli is the requirement for a particular KS enzyme, β-ketoacyl-ACP synthase, the product of the FabB gene. This is the enzyme that carries out condensation of a fatty acid, linked to a cysteine residue at the active site (by a thio-ester bond), with a malonyl-ACP. In the multi-step reaction, CO₂ is released and the linear chain is extended by two carbons. It is believed that only this KS can extend a carbon chain that contains a double bond. This extension occurs only when the double bond is in the cis configuration; if it is in the trans configuration, the double bond is reduced by enoyl-ACP reductase (ER) prior to elongation (Heath et al., 1996, supra). All of the PUFA-PKS systems characterized so far have two KS domains, one of which shows greater homology to the FabB-like KS of E. coli than the other. Again, without being bound by theory, the present inventors believe that in PUFA-PKS systems, the specificities and interactions of the DH (FabA-like) and KS (FabB-like) enzymatic domains determine the number and placement of cis double bonds in the end products. Because the number of 2-carbon elongation reactions is greater than the number of double bonds present in the PUFA-PKS end products, it can be determined that in some extension cycles complete reduction occurs. Thus the DH and KS domains can be used as targets for alteration of the DHA/DPA ratio or ratios of other long chain fatty acids. These can be modified and/or evaluated by introduction of homologous domains from other systems or by mutagenesis of these gene fragments.

In another embodiment, the ER (enoyl-ACP reductase—an enzyme which reduces the trans-double bond in the fatty acyl-ACP resulting in fully saturated carbons) domains can be modified or substituted to change the type of product made by the PKS system. For example, the present inventors know that Schizochytrium PUFA-PKS system differs from the previously described bacterial systems in that it has two (rather than one) ER domains. Without being bound by theory, the present inventors believe these ER domains can strongly influence the resulting PKS production product. The resulting PKS product could be changed by separately knocking out the individual domains or by modifying their nucleotide sequence or by substitution of ER domains from other organisms.

In another embodiment, nucleic acid molecules encoding proteins or domains that are not part of a PKS system, but which affect a PKS system, can be introduced into an organism. For example, all of the PUFA PKS systems described above contain multiple, tandem, ACP domains. ACP (as a separate protein or as a domain of a larger protein) requires attachment of a phosphopantetheine cofactor to produce the active, holo-ACP. Attachment of phosphopantetheine to the apo-ACP is carried out by members of the superfamily of enzymes—the phosphopantetheinyl transferases (PPTase) (Lambalot R. H., et al., Chemistry and Biology, 3, 923 (1996)).

By analogy to other PKS and FAS systems, the present inventors presume that activation of the multiple ACP domains present in the Schizochytrium ORFA protein is carried out by a specific, endogenous, PPTase. The gene encoding this presumed PPTase has not yet been identified in Schizochytrium. If such a gene is present in Schizochytrium, one can envision several approaches that could be used in an attempt to identify and clone it. These could include (but would not be limited to): generation and partial sequencing of a cDNA library prepared from actively growing Schizochytrium cells (note, one sequence was identified in the currently available Schizochytrium cDNA library set which showed homology to PPTase's; however, it appears to be part of a multidomain FAS protein, and as such may not encode the desired OrfA specific PPTase); use of degenerate oligonucleotide primers designed using amino acid motifs present in many PPTase's in PCR reactions (to obtain a nucleic acid probe molecule to screen genomic or cDNA libraries); genetic approaches based on protein-protein interactions (e.g. a yeast two-hybrid system) in which the ORFA-ACP domains would be used as a “bait” to find a “target” (i.e. the PPTase); and purification and partial sequencing of the enzyme itself as a means to generate a nucleic acid probe for screening of genomic or cDNA libraries.

It is also conceivable that a heterologous PPTase may be capable of activating the Schizochytrium ORFA ACP domains. It has been shown that some PPTases, for example the sfp enzyme of Bacillus subtilis (Lambalot et al., supra) and the svp enzyme of Streptomyces verticillus (Sanchez et al., 2001, Chemistry & Biology 8:725-738), have a broad substrate tolerance. These enzymes can be tested to see if they will activate the Schizochytrium ACP domains. Also, a recent publication described the expression of a fungal PKS protein in tobacco (Yalpani et al., 2001, The Plant Cell 13:1401-1409). Products of the introduced PKS system (encoded by the 6-methylsalicyclic acid synthase gene of Penicillium patulum) were detected in the transgenic plant, even though the corresponding fungal PPTase was not present in those plants. This suggested that an endogenous plant PPTase(s) recognized and activated the fungal PKS ACP domain. Of relevance to this observation, the present inventors have identified two sequences (genes) in the Arabidopsis whole genome database that are likely to encode PPTases. These sequences (GenBank Accession numbers; AAG51443 and AAC05345) are currently listed as encoding “Unknown Proteins”. They can be identified as putative PPTases based on the presence in the translated protein sequences of several signature motifs including; G(I/V)D and WxxKE(A/S)xxK (SEQ ID NO:33), (listed in Lambalot et al., 1996 as characteristic of all PPTases). In addition, these two putative proteins contain two additional motifs typically found in PPTases typically associated with PKS and non-ribosomal peptide synthesis systems; i.e., FN(IL/V)SHS (SEQ ID NO:34) and (I/V/L)G(IL/V)D(IL/V) (SEQ ID NO:35). Furthermore, these motifs occur in the expected relative positions in the protein sequences. It is likely that homologues of the Arabidopsis genes are present in other plants, such as tobacco. Again, these genes can be cloned and expressed to see if the enzymes they encode can activate the Schizochytrium ORFA ACP domains, or alternatively, OrfA could be expressed directly in the transgenic plant (either targeted to the plastid or the cytoplasm).

Another heterologous PPTase which may recognize the ORFA ACP domains as substrates is the Het I protein of Nostoc sp. PCC 7120 (formerly called Anabaena sp. PCC 7120). As noted in U.S. Pat. No. 6,140,486, several of the PUFA-PKS genes of Shewanella showed a high degree of homology to protein domains present in a PKS cluster found in Nostoc (FIG. 2 of that patent). This Nostoc PKS system is associated with the synthesis of long chain (C26 or C28) hydroxy fatty acids that become esterified to sugar moieties and form a part of the heterocyst cell wall. These Nostoc PKS domains are also highly homologous to the domains found in Orfs B and C of the Schizochytrium PKS proteins (i.e. the same ones that correspond to those found in the Shewanella PKS proteins). Until very recently, none of the Nostoc PKS domains present in the GenBank databases showed high homology to any of the domains of Schizochytrium OrfA (or the homologous Shewanella Orf 5 protein). However, the complete genome of Nostoc has recently been sequenced and as a result, the sequence of the region just upstream of the PKS gene cluster is now available. In this region are three Orfs that show homology to the domains (KS, MAT, ACP and KR) of OrfA (see FIG. 3). Included in this set are two ACP domains, both of which show high homology to the ORFA ACP domains. At the end of the Nostoc PKS cluster is the gene that encodes the Het I PPTase. Previously, it was not obvious what the substrate of the Het I enzyme could be, however the presence of tandem ACP domains in the newly identified Orf (Hgl E) of the cluster strongly suggests to the present inventors that it is those ACPs. The homology of the ACP domains of Schizochytrium and Nostoc, as well as the tandem arrangement of the domains in both proteins, makes Het I a likely candidate for heterologous activation of the Schizochytrium ORFA ACPs. The present inventors are believed to be the first to recognize and contemplate this use for Nostoc Het I PPTase.

As indicated in Metz et al., 2001, supra, one novel feature of the PUFA PKS systems is the presence of two dehydratase domains, both of which show homology to the FabA proteins of E. coli. With the availability of the new Nostoc PKS gene sequences mentioned above, one can now compare the two systems and their products. The sequence of domains in the Nostoc cluster (from HglE to Het I) as the present inventors have defined them is (see FIG. 3):

KS-MAT-2×ACP, KR, KS, CLF-AT, ER (HetM, HetN) HetI

In the Schizochytrium PUFA-PKS Orfs A,B&C the sequence (OrfA-B-C) is:

KS-MAT-9×ACP-KR KS-CLF-AT-ER DH-DH-ER

One can see the correspondence of the domains sequence (there is also a high amino acid sequence homology). The product of the Nostoc PKS system is a long chain hydroxy fatty acid (C26 or C28 with one or two hydroxy groups) that contains no double bonds (cis or trans). The product of the Schizochytrium PKS system is a long chain polyunsaturated fatty acid (C22, with 5 or 6 double bonds—all cis). An obvious difference between the two domain sets is the presence of the two DH domains in the Schizochytrium proteins—just the domains implicated in the formation of the cis double bonds of DHA and DPA (presumably HetM and HetN in the Nostoc system are involved in inclusion of the hydroxyl groups and also contain a DH domain whose origin differs from the those found in the PUFA). Also, the role of the duplicated ER domain in the Schizochytrium Orfs B and C is not known (the second ER domain in is not present other characterized PUFA PKS systems). The amino acid sequence homology between the two sets of domains implies an evolutionary relationship. One can conceive of the PUFA PKS gene set being derived from (in an evolutionary sense) an ancestral Nostoc-like PKS gene set by incorporation of the DH (FabA-like) domains. The addition of the DH domains would result in the introduction of cis double bonds in the new PKS end product structure.

The comparisons of the Schizochytrium and Nostoc PKS domain structures as well as the comparison of the domain organization between the Schizochytrium and Shewanella PUFA-PKS proteins demonstrate nature's ability to alter domain order as well as incorporate new domains to create novel end products. In addition, the genes can now be manipulated in the laboratory to create new products. The implication from these observations is that it should be possible to continue to manipulate the systems in either a directed or random way to influence the end products. For example, in a preferred embodiment, one could envision substituting one of the DH (FabA-like) domains of the PUFA-PKS system for a DH domain that did not posses isomerization activity, potentially creating a molecule with a mix of cis- and trans-double bonds. The current products of the Schizochytrium PUFA PKS system are DHA and DPA (C22:5 ω6). If one manipulated the system to produce C20 fatty acids, one would expect the products to be EPA and ARA (C20:4 ω6). This could provide a new source for ARA. One could also substitute domains from related PUFA-PKS systems that produced a different DHA to DPA ratio—for example by using genes from Thraustochytrium 23B (the PUFA PKS system of which is identified for the first time herein).

Additionally, one could envision specifically altering one of the ER domains (e.g. removing, or inactivating) in the Schizochytrium PUFA PKS system (other PUFA PKS systems described so far do not have two ER domains) to determine its effect on the end product profile. Similar strategies could be attempted in a directed manner for each of the distinct domains of the PUFA-PKS proteins using more or less sophisticated approaches. Of course one would not be limited to the manipulation of single domains. Finally, one could extend the approach by mixing domains from the PUFA-PKS system and other PKS or FAS systems (e.g., type I, type II, modular) to create an entire range of new end products. For example, one could introduce the PUFA-PKS DH domains into systems that do not normally incorporate cis double bonds into their end products.

Accordingly, encompassed by the present invention are methods to genetically modify microbial or plant cells by: genetically modifying at least one nucleic acid sequence in the organism that encodes an amino acid sequence having the biological activity of at least one functional domain of a non-bacterial PUFA PKS system according to the present invention, and/or expressing at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding such amino acid sequence. Various embodiments of such sequences, methods to genetically modify an organism, and specific modifications have been described in detail above. Typically, the method is used to produce a particular genetically modified organism that produces a particular bioactive molecule or molecules.

One embodiment of the present invention relates to a recombinant host cell which has been modified to express a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PKS catalyzes both iterative and non-iterative enzymatic reactions, and wherein the PUFA PKS system comprises: (a) at least two enoyl ACP-reductase (ER) domains; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. In one embodiment, the PUFA PKS system is a eukaryotic PUFA PKS system. In a preferred embodiment, the PUFA PKS system is an algal PUFA PKS system. In a more preferred embodiment, the PUFA PKS system is a Thraustochytriales PUFA PKS system. Such PUFA PKS systems can include, but are not limited to, a Schizochytrium PUFA PKS system, and a Thraustochytrium PUFA PKS system. In one embodiment, the PUFA PKS system can be expressed in a prokaryotic host cell. In another embodiment, the PUFA PKS system can be expressed in a eukaryotic host cell.

Another embodiment of the present invention relates to a recombinant host cell which has been modified to express a non-bacterial PUFA PKS system, wherein the PKS system catalyzes both iterative and non-iterative enzymatic reactions, and wherein the non-bacterial PUFA PKS system comprises at least the following biologically active domains: (a) at least one enoyl ACP-reductase (ER) domain; (b) multiple acyl carrier protein (ACP) domains (at least four); (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain.

One aspect of this embodiment of the invention relates to a method to produce a product containing at least one PUFA, comprising growing a plant comprising any of the recombinant host cells described above, wherein the recombinant host cell is a plant cell, under conditions effective to produce the product. Another aspect of this embodiment of the invention relates to a method to produce a product containing at least one PUFA, comprising culturing a culture containing any of the recombinant host cells described above, wherein the host cell is a microbial cell, under conditions effective to produce the product. In a preferred embodiment, the PKS system in the host cell catalyzes the direct production of triglycerides.

Another embodiment of the present invention relates to a microorganism comprising a non-bacterial, polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PKS catalyzes both iterative and non-iterative enzymatic reactions, and wherein the PUFA PKS system comprises: (a) at least two enoyl ACP-reductase (ER) domains; (b) at least six acyl carrier protein (ACP) domains; (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain. Preferably, the microorganism is a non-bacterial microorganism and more preferably, a eukaryotic microorganism.

Yet another embodiment of the present invention relates to a microorganism comprising a non-bacterial, polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system, wherein the PKS catalyzes both iterative and non-iterative enzymatic reactions, and wherein the PUFA PKS system comprises: (a) at least one enoyl ACP-reductase (ER) domain; (b) multiple acyl carrier protein (ACP) domains (at least four); (c) at least two β-keto acyl-ACP synthase (KS) domains; (d) at least one acyltransferase (AT) domain; (e) at least one ketoreductase (KR) domain; (f) at least two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains; (g) at least one chain length factor (CLF) domain; and (h) at least one malonyl-CoA:ACP acyltransferase (MAT) domain.

In one embodiment of the present invention, it is contemplated that a mutagenesis program could be combined with a selective screening process to obtain bioactive molecules of interest. This would include methods to search for a range of bioactive compounds. This search would not be restricted to production of those molecules with cis double bonds. The mutagenesis methods could include, but are not limited to: chemical mutagenesis, gene shuffling, switching regions of the genes encoding specific enzymatic domains, or mutagenesis restricted to specific regions of those genes, as well as other methods.

For example, high throughput mutagenesis methods could be used to influence or optimize production of the desired bioactive molecule. Once an effective model system has been developed, one could modify these genes in a high throughput manner. Utilization of these technologies can be envisioned on two levels. First, if a sufficiently selective screen for production of a product of interest (e.g., ARA) can be devised, it could be used to attempt to alter the system to produce this product (e.g., in lieu of, or in concert with, other strategies such as those discussed above). Additionally, if the strategies outlined above resulted in a set of genes that did produce the product of interest, the high throughput technologies could then be used to optimize the system. For example, if the introduced domain only functioned at relatively low temperatures, selection methods could be devised to permit removing that limitation. In one embodiment of the invention, screening methods are used to identify additional non-bacterial organisms having novel PKS systems similar to the PUFA PKS system of Schizochytrium, as described herein (see above). Homologous PKS systems identified in such organisms can be used in methods similar to those described herein for the Schizochytrium, as well as for an additional source of genetic material from which to create, further modify and/or mutate a PKS system for expression in that microorganism, in another microorganism, or in a higher plant, to produce a variety of compounds.

It is recognized that many genetic alterations, either random or directed, which one may introduce into a native (endogenous, natural) PKS system, will result in an inactivation of enzymatic functions. A preferred embodiment of the invention includes a system to select for only those modifications that do not block the ability of the PKS system to produce a product. For example, the FabB-strain of E. coli is incapable of synthesizing unsaturated fatty acids and requires supplementation of the medium with fatty acids that can substitute for its normal unsaturated fatty acids in order to grow (see Metz et al., 2001, supra). However, this requirement (for supplementation of the medium) can be removed when the strain is transformed with a functional PUFA-PKS system (i.e. one that produces a PUFA product in the E. coli host—see (Metz et al., 2001, supra, FIG. 2A). The transformed FabB-strain now requires a functional PUFA-PKS system (to produce the unsaturated fatty acids) for growth without supplementation. The key element in this example is that production of a wide range of unsaturated fatty acid will suffice (even unsaturated fatty acid substitutes such as branched chain fatty acids). Therefore, in another preferred embodiment of the invention, one could create a large number of mutations in one or more of the PUFA PKS genes disclosed herein, and then transform the appropriately modified FabB-strain (e.g. create mutations in an expression construct containing an ER domain and transform a FabB-strain having the other essential domains on a separate plasmid—or integrated into the chromosome) and select only for those transformants that grow without supplementation of the medium (i.e., that still possessed an ability to produce a molecule that could complement the FabB-defect). Additional screens could be developed to look for particular compounds (e.g. use of GC for fatty acids) being produced in this selective subset of an active PKS system. One could envision a number of similar selective screens for bioactive molecules of interest.

As described above, in one embodiment of the present invention, a genetically modified microorganism or plant includes a microorganism or plant which has an enhanced ability to synthesize desired bioactive molecules (products) or which has a newly introduced ability to synthesize specific products (e.g., to synthesize a specific antibiotic). According to the present invention, “an enhanced ability to synthesize” a product refers to any enhancement, or up-regulation, in a pathway related to the synthesis of the product such that the microorganism or plant produces an increased amount of the product (including any production of a product where there was none before) as compared to the wild-type microorganism or plant, cultured or grown, under the same conditions. Methods to produce such genetically modified organisms have been described in detail above.

One embodiment of the present invention is a method to produce desired bioactive molecules (also referred to as products or compounds) by growing or culturing a genetically modified microorganism or plant of the present invention (described in detail above). Such a method includes the step of culturing in a fermentation medium or growing in a suitable environment, such as soil, a microorganism or plant, respectively, that has a genetic modification as described previously herein and in accordance with the present invention. In a preferred embodiment, method to produce bioactive molecules of the present invention includes the step of culturing under conditions effective to produce the bioactive molecule a genetically modified organism that expresses a PKS system comprising at least one biologically active domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. In this preferred aspect, at least one domain of the PUFA PKS system is encoded by a nucleic acid sequence selected from the group consisting of: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by the novel screening method of the present invention (described above in detail); (c) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof, (d) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof, (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and, (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. In this preferred aspect of the method, the organism is genetically modified to affect the activity of the PKS system (described in detail above). Preferred host cells for genetic modification related to the PUFA PKS system of the invention are described above.

In the method of production of desired bioactive compounds of the present invention, a genetically modified microorganism is cultured or grown in a suitable medium, under conditions effective to produce the bioactive compound. An appropriate, or effective, medium refers to any medium in which a genetically modified microorganism of the present invention, when cultured, is capable of producing the desired product. Such a medium is typically an aqueous medium comprising assimilable carbon, nitrogen and phosphate sources. Such a medium can also include appropriate salts, minerals, metals and other nutrients. Microorganisms of the present invention can be cultured in conventional fermentation bioreactors. The microorganisms can be cultured by any fermentation process which includes, but is not limited to, batch, fed-batch, cell recycle, and continuous fermentation. Preferred growth conditions for potential host microorganisms according to the present invention are well known in the art. The desired bioactive molecules produced by the genetically modified microorganism can be recovered from the fermentation medium using conventional separation and purification techniques. For example, the fermentation medium can be filtered or centrifuged to remove microorganisms, cell debris and other particulate matter, and the product can be recovered from the cell-free supernatant by conventional methods, such as, for example, ion exchange, chromatography, extraction, solvent extraction, membrane separation, electrodialysis, reverse osmosis, distillation, chemical derivatization and crystallization. Alternatively, microorganisms producing the desired compound, or extracts and various fractions thereof, can be used without removal of the microorganism components from the product.

In the method for production of desired bioactive compounds of the present invention, a genetically modified plant is cultured in a fermentation medium or grown in a suitable medium such as soil. An appropriate, or effective, fermentation medium has been discussed in detail above. A suitable growth medium for higher plants includes any growth medium for plants, including, but not limited to, soil, sand, any other particulate media that support root growth (e.g. vermiculite, perlite, etc.) or Hydroponic culture, as well as suitable light, water and nutritional supplements which optimize the growth of the higher plant. The genetically modified plants of the present invention are engineered to produce significant quantities of the desired product through the activity of the PKS system that is genetically modified according to the present invention. The compounds can be recovered through purification processes which extract the compounds from the plant. In a preferred embodiment, the compound is recovered by harvesting the plant. In this embodiment, the plant can be consumed in its natural state or further processed into consumable products.

As described above, a genetically modified microorganism useful in the present invention can, in one aspect, endogenously contain and express a PUFA PKS system, and the genetic modification can be a genetic modification of one or more of the functional domains of the endogenous PUFA PKS system, whereby the modification has some effect on the activity of the PUFA PKS system. In another aspect, such an organism can endogenously contain and express a PUFA PKS system, and the genetic modification can be an introduction of at least one exogenous nucleic acid sequence (e.g., a recombinant nucleic acid molecule), wherein the exogenous nucleic acid sequence encodes at least one biologically active domain or protein from a second PKS system and/or a protein that affects the activity of said PUFA PKS system (e.g., a phosphopantetheinyl transferases (PPTase), discussed below). In yet another aspect, the organism does not necessarily endogenously (naturally) contain a PUFA PKS system, but is genetically modified to introduce at least one recombinant nucleic acid molecule encoding an amino acid sequence having the biological activity of at least one domain of a PUFA PKS system. In this aspect, PUFA PKS activity is affected by introducing or increasing PUFA PKS activity in the organism. Various embodiments associated with each of these aspects have been discussed in detail above.

In one embodiment of the method to produce bioactive compounds, the genetic modification changes at least one product produced by the endogenous PKS system, as compared to a wild-type organism.

In another embodiment, the organism endogenously expresses a PKS system comprising the at least one biologically active domain of the PUFA PKS system, and the genetic modification comprises transfection of the organism with a recombinant nucleic acid molecule selected from the group consisting of: a recombinant nucleic acid molecule encoding at least one biologically active domain from a second PKS system and a recombinant nucleic acid molecule encoding a protein that affects the activity of the PUFA PKS system. In this embodiment, the genetic modification preferably changes at least one product produced by the endogenous PKS system, as compared to a wild-type organism. A second PKS system can include another PUFA PKS system (bacterial or non-bacterial), a type I PKS system, a type II PKS system, and/or a modular PKS system. Examples of proteins that affect the activity of a PKS system have been described above (e.g., PPTase).

In another embodiment, the organism is genetically modified by transfection with a recombinant nucleic acid molecule encoding the at least one domain of the polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system. Such recombinant nucleic acid molecules have been described in detail previously herein.

In another embodiment, the organism endogenously expresses a non-bacterial PUFA PKS system, and the genetic modification comprises substitution of a domain from a different PKS system for a nucleic acid sequence encoding at least one domain of the non-bacterial PUFA PKS system. In another embodiment, the organism endogenously expresses a non-bacterial PUFA PKS system that has been modified by transfecting the organism with a recombinant nucleic acid molecule encoding a protein that regulates the chain length of fatty acids produced by the PUFA PKS system. In one aspect, the recombinant nucleic acid molecule encoding a protein that regulates the chain length of fatty acids replaces a nucleic acid sequence encoding a chain length factor in the non-bacterial PUFA PKS system. In another aspect, the protein that regulates the chain length of fatty acids produced by the PUFA PKS system is a chain length factor. In another aspect, the protein that regulates the chain length of fatty acids produced by the PUFA PKS system is a chain length factor that directs the synthesis of C20 units.

In another embodiment, the organism expresses a non-bacterial PUFA PKS system comprising a genetic modification in a domain selected from the group consisting of a domain encoding β-hydroxy acyl-ACP dehydrase (DH) and a domain encoding β-ketoacyl-ACP synthase (KS), wherein the modification alters the ratio of long chain fatty acids produced by the PUFA PKS system as compared to in the absence of the modification. In one aspect of this embodiment, the modification is selected from the group consisting of a deletion of all or a part of the domain, a substitution of a homologous domain from a different organism for the domain, and a mutation of the domain.

In another embodiment, the organism expresses a non-bacterial PUFA PKS system comprising a modification in an enoyl-ACP reductase (ER) domain, wherein the modification results in the production of a different compound as compared to in the absence of the modification. In one aspect of this embodiment, the modification is selected from the group consisting of a deletion of all or a part of the ER domain, a substitution of an ER domain from a different organism for the ER domain, and a mutation of the ER domain.

In one embodiment of the method to produce a bioactive molecule, the organism produces a polyunsaturated fatty acid (PUFA) profile that differs from the naturally occurring organism without a genetic modification.

Many other genetic modifications useful for producing bioactive molecules will be apparent to those of skill in the art, given the present disclosure, and various other modifications have been discussed previously herein. The present invention contemplates any genetic modification related to a PUFA PKS system as described herein which results in the production of a desired bioactive molecule.

Bioactive molecules, according to the present invention, include any molecules (compounds, products, etc.) that have a biological activity, and that can be produced by a PKS system that comprises at least one amino acid sequence having a biological activity of at least one functional domain of a non-bacterial PUFA PKS system as described herein. Such bioactive molecules can include, but are not limited to: a polyunsaturated fatty acid (PUFA), an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti-Heliobactor pylori drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. One advantage of the non-bacterial PUFA PKS system of the present invention is the ability of such a system to introduce carbon-carbon double bonds in the cis configuration, and molecules including a double bond at every third carbon. This ability can be utilized to produce a variety of compounds.

Preferably, bioactive compounds of interest are produced by the genetically modified microorganism in an amount that is greater than about 0.05%, and preferably greater than about 0.1%, and more preferably greater than about 0.25%, and more preferably greater than about 0.5%, and more preferably greater than about 0.75%, and more preferably greater than about 1%, and more preferably greater than about 2.5%, and more preferably greater than about 5%, and more preferably greater than about 10%, and more preferably greater than about 15%, and even more preferably greater than about 20% of the dry weight of the microorganism. For lipid compounds, preferably, such compounds are produced in an amount that is greater than about 5% of the dry weight of the microorganism. For other bioactive compounds, such as antibiotics or compounds that are synthesized in smaller amounts, those strains possessing such compounds at of the dry weight of the microorganism are identified as predictably containing a novel PKS system of the type described above. In some embodiments, particular bioactive molecules (compounds) are secreted by the microorganism, rather than accumulating. Therefore, such bioactive molecules are generally recovered from the culture medium and the concentration of molecule produced will vary depending on the microorganism and the size of the culture.

One embodiment of the present invention relates to a method to modify an endproduct containing at least one fatty acid, comprising adding to said endproduct an oil produced by a recombinant host cell that expresses at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The PUFA PKS system is any non-bacterial PUFA PKS system, and preferably, is selected from the group of: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by the novel screening method disclosed herein; (c) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof, (d) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof; (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and, (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system. Variations of these nucleic acid sequences have been described in detail above.

Preferably, the endproduct is selected from the group consisting of a food, a dietary supplement, a pharmaceutical formulation, a humanized animal milk, and an infant formula. Suitable pharmaceutical formulations include, but are not limited to, an anti-inflammatory formulation, a chemotherapeutic agent, an active excipient, an osteoporosis drug, an anti-depressant, an anti-convulsant, an anti-Heliobactor pylori drug, a drug for treatment of neurodegenerative disease, a drug for treatment of degenerative liver disease, an antibiotic, and a cholesterol lowering formulation. In one embodiment, the endproduct is used to treat a condition selected from the group consisting of: chronic inflammation, acute inflammation, gastrointestinal disorder, cancer, cachexia, cardiac restenosis, neurodegenerative disorder, degenerative disorder of the liver, blood lipid disorder, osteoporosis, osteoarthritis, autoimmune disease, preeclampsia, preterm birth, age related maculopathy, pulmonary disorder, and peroxisomal disorder.

Suitable food products include, but are not limited to, fine bakery wares, bread and rolls, breakfast cereals, processed and unprocessed cheese, condiments (ketchup, mayonnaise, etc.), dairy products (milk, yogurt), puddings and gelatine desserts, carbonated drinks, teas, powdered beverage mixes, processed fish products, fruit-based drinks, chewing gum, hard confectionery, frozen dairy products, processed meat products, nut and nut-based spreads, pasta, processed poultry products, gravies and sauces, potato chips and other chips or crisps, chocolate and other confectionery, soups and soup mixes, soya based products (milks, drinks, creams, whiteners), vegetable oil-based spreads, and vegetable-based drinks.

Yet another embodiment of the present invention relates to a method to produce a humanized animal milk. This method includes the steps of genetically modifying milk-producing cells of a milk-producing animal with at least one recombinant nucleic acid molecule comprising a nucleic acid sequence encoding at least one biologically active domain of a PUFA PKS system. The PUFA PKS system is a non-bacterial PUFA PKS system, and preferably, the at least one domain of the PUFA PKS system is encoded by a nucleic acid sequence selected from the group consisting of: (a) a nucleic acid sequence encoding at least one domain of a polyunsaturated fatty acid (PUFA) polyketide synthase (PKS) system from a Thraustochytrid microorganism; (b) a nucleic acid sequence encoding at least one domain of a PUFA PKS system from a microorganism identified by the novel screening method described previously herein; (c) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, and biologically active fragments thereof, (d) a nucleic acid sequence encoding an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, SEQ ID NO:32, and biologically active fragments thereof; (e) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to at least 500 consecutive amino acids of an amino acid sequence selected from the group consisting of: SEQ ID NO:2, SEQ ID NO:4, and SEQ ID NO:6; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system; and/or (f) a nucleic acid sequence encoding an amino acid sequence that is at least about 60% identical to an amino acid sequence selected from the group consisting of: SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:13, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, SEQ ID NO:26, SEQ ID NO:28, SEQ ID NO:30, and SEQ ID NO:32; wherein the amino acid sequence has a biological activity of at least one domain of a PUFA PKS system.

Methods to genetically modify a host cell and to produce a genetically modified non-human, milk-producing animal, are known in the art. Examples of host animals to modify include cattle, sheep, pigs, goats, yaks, etc., which are amenable to genetic manipulation and cloning for rapid expansion of a transgene expressing population. For animals, PKS-like transgenes can be adapted for expression in target organelles, tissues and body fluids through modification of the gene regulatory regions. Of particular interest is the production of PUFAs in the breast milk of the host animal.

The following examples are provided for the purpose of illustration and are not intended to limit the scope of the present invention.

EXAMPLES Example 1

The following example describes the further analysis of PKS related sequences from Schizochytrium.

The present inventors have sequenced the genomic DNA including the entire length of all three open reading frames (Orfs) in the Schizochytrium PUFA PKS system using the general methods outlined in Examples 8 and 9 from PCT Publication No. WO 0042195 and U.S. application Ser. No. 09/231,899. The biologically active domains in the Schizochytrium PKS proteins are depicted graphically in FIG. 1. The domain structure of the Schizochytrium PUFA PKS system is described more particularly as follows.

Open Reading Frame A (OrfA):

The complete nucleotide sequence for OrfA is represented herein as SEQ ID NO:1. OrfA is a 8730 nucleotide sequence (not including the stop codon) which encodes a 2910 amino acid sequence, represented herein as SEQ ID NO:2. Within OrfA are twelve domains:

(a) one β-keto acyl-ACP synthase (KS) domain;

(b) one malonyl-CoA:ACP acyltransferase (MAT) domain;

(c) nine acyl carrier protein (ACP) domains;

(d) one ketoreductase (KR) domain.

The domains contained within OrfA have been determined based on:

(1) results of an analysis with Pfam program (Pfam is a database of multiple alignments of protein domains or conserved protein regions. The alignments represent some evolutionary conserved structure that has implications for the protein's function. Profile hidden Markov models (profile HMMs) built from the Pfam alignments can be very useful for automatically recognizing that a new protein belongs to an existing protein family, even if the homology is weak. Unlike standard pairwise alignment methods (e.g. BLAST, FASTA), Pfam HMMs deal sensibly with multidomain proteins. The reference provided for the Pfam version used is: Bateman A, Birney E, Cerruti L, Durbin R, Etwiller L, Eddy S R, Griffiths-Jones S, Howe K L, Marshall M, Sonnhammer E L (2002) Nucleic Acids Research 30(1):276-280); and/or

(2) homology comparison to bacterial PUFA-PKS systems (e.g., Shewanella) using a BLAST 2.0 Basic BLAST homology search using blastp for amino acid searches with standard default parameters, wherein the query sequence is filtered for low complexity regions by default (described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. & Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402, incorporated herein by reference in its entirety).

Sequences provided for individual domains are believed to contain the full length of the sequence encoding a functional domain, and may contain additional flanking sequence within the Orf.

ORFA-KS

The first domain in OrfA is a KS domain, also referred to herein as ORFA-KS. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 40 of SEQ ID NO:1 (OrfA) to an ending point of between about positions 1428 and 1500 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-KS domain is represented herein as SEQ ID NO:7 (positions 1-1500 of SEQ ID NO:1). The amino acid sequence containing the KS domain spans from a starting point of between about positions 1 and 14 of SEQ ID NO:2 (ORFA) to an ending point of between about positions 476 and 500 of SEQ ID NO:2. The amino acid sequence containing the ORFA-KS domain is represented herein as SEQ ID NO:8 (positions 1-500 of SEQ ID NO:2). It is noted that the ORFA-KS domain contains an active site motif: DXAC* (*acyl binding site C₂₁₅).

ORFA-MAT

The second domain in OrfA is a MAT domain, also referred to herein as ORFA-MAT. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1723 and 1798 of SEQ ID NO:1 (OrfA) to an ending point of between about positions 2805 and 3000 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-MAT domain is represented herein as SEQ ID NO:9 (positions 1723-3000 of SEQ ID NO:1). The amino acid sequence containing the MAT domain spans from a starting point of between about positions 575 and 600 of SEQ ID NO:2 (ORFA) to an ending point of between about positions 935 and 1000 of SEQ ID NO:2. The amino acid sequence containing the ORFA-MAT domain is represented herein as SEQ ID NO:10 (positions 575-1000 of SEQ ID NO:2). It is noted that the ORFA-MAT domain contains an active site motif: GHS*XG (*acyl binding site S₇₀₆), represented herein as SEQ ID NO:11.

ORFA-ACP # 1-9

Domains 3-11 of OrfA are nine tandem ACP domains, also referred to herein as ORFA-ACP (the first domain in the sequence is ORFA-ACP1, the second domain is ORFA-ACP2, the third domain is ORFA-ACP3, etc.). The first ACP domain, ORFA-ACP1, is contained within the nucleotide sequence spanning from about position 3343 to about position 3600 of SEQ ID NO:1 (OrfA). The nucleotide sequence containing the sequence encoding the ORFA-ACP1 domain is represented herein as SEQ ID NO:12 (positions 3343-3600 of SEQ ID NO:1). The amino acid sequence containing the first ACP domain spans from about position 1115 to about position 1200 of SEQ ID NO:2. The amino acid sequence containing the ORFA-ACP1 domain is represented herein as SEQ ID NO:13 (positions 1115-1200 of SEQ ID NO:2). It is noted that the ORFA-ACP1 domain contains an active site motif: LGIDS* (*pantetheine binding motif S₁₁₅₇), represented herein by SEQ ID NO:14. The nucleotide and amino acid sequences of all nine ACP domains are highly conserved and therefore, the sequence for each domain is not represented herein by an individual sequence identifier. However, based on this information, one of skill in the art can readily determine the sequence for each of the other eight ACP domains. The repeat interval for the nine domains is approximately about 110 to about 330 nucleotides of SEQ ID NO:1.

All nine ACP domains together span a region of OrfA of from about position 3283 to about position 6288 of SEQ ID NO:1, which corresponds to amino acid positions of from about 1095 to about 2096 of SEQ ID NO:2. This region includes the linker segments between individual ACP domains. Each of the nine ACP domains contains a pantetheine binding motif LGIDS* (represented herein by SEQ ID NO:14), wherein * is the pantetheine binding site S. At each end of the ACP domain region and between each ACP domain is a region that is highly enriched for proline (P) and alanine (A), which is believed to be a linker region. For example, between ACP domains 1 and 2 is the sequence: APAPVKAAAPAAPVASAPAPA, represented herein as SEQ ID NO:15.

ORFA-KR

Domain 12 in OrfA is a KR domain, also referred to herein as ORFA-KR. This domain is contained within the nucleotide sequence spanning from a starting point of about position 6598 of SEQ ID NO:1 to an ending point of about position 8730 of SEQ ID NO:1. The nucleotide sequence containing the sequence encoding the ORFA-KR domain is represented herein as SEQ ID NO:17 (positions 6598-8730 of SEQ ID NO:1). The amino acid sequence containing the KR domain spans from a starting point of about position 2200 of SEQ ID NO:2 (ORFA) to an ending point of about position 2910 of SEQ ID NO:2. The amino acid sequence containing the ORFA-KR domain is represented herein as SEQ ID NO:18 (positions 2200-2910 of SEQ ID NO:2). Within the KR domain is a core region with homology to short chain aldehyde-dehydrogenases (KR is a member of this family). This core region spans from about position 7198 to about position 7500 of SEQ ID NO:1, which corresponds to amino acid positions 2400-2500 of SEQ ID NO:2.

Open Reading Frame B (OrfB):

The complete nucleotide sequence for OrfB is represented herein as SEQ ID NO:3. OrfB is a 6177 nucleotide sequence (not including the stop codon) which encodes a 2059 amino acid sequence, represented herein as SEQ ID NO:4. Within OrfB are four domains:

(a) β-keto acyl-ACP synthase (KS) domain;

(b) one chain length factor (CLF) domain;

(c) one acyl transferase (AT) domain;

(d) one enoyl ACP-reductase (ER) domain.

The domains contained within ORFB have been determined based on: (1) results of an analysis with Pfam program, described above; and/or (2) homology comparison to bacterial PUFA-PKS systems (e.g., Shewanella) using a BLAST 2.0 Basic BLAST homology search, also described above. Sequences provided for individual domains are believed to contain the full length of the sequence encoding a functional domain, and may contain additional flanking sequence within the Orf.

ORFB-KS

The first domain in OrfB is a KS domain, also referred to herein as ORFB-KS. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 43 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 1332 and 1350 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-KS domain is represented herein as SEQ ID NO:19 (positions 1-1350 of SEQ ID NO:3). The amino acid sequence containing the KS domain spans from a starting point of between about positions 1 and 15 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 444 and 450 of SEQ ID NO:4. The amino acid sequence containing the ORFB-KS domain is represented herein as SEQ ID NO:20 (positions 1-450 of SEQ ID NO:4). It is noted that the ORFB-KS domain contains an active site motif: DXAC* (*acyl binding site C₁₉₆).

ORFB-CLF

The second domain in OrfB is a CLF domain, also referred to herein as ORFB-CLF. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1378 and 1402 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 2682 and 2700 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-CLF domain is represented herein as SEQ ID NO:21 (positions 1378-2700 of SEQ ID NO:3). The amino acid sequence containing the CLF domain spans from a starting point of between about positions 460 and 468 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 894 and 900 of SEQ ID NO:4. The amino acid sequence containing the ORFB-CLF domain is represented herein as SEQ ID NO:22 (positions 460-900 of SEQ ID NO:4). It is noted that the ORFB-CLF domain contains a KS active site motif without the acyl-binding cysteine.

ORFB-AT

The third domain in OrfB is an AT domain, also referred to herein as ORFB-AT. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 2701 and 3598 of SEQ ID NO:3 (OrfB) to an ending point of between about positions 3975 and 4200 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-AT domain is represented herein as SEQ ID NO:23 (positions 2701-4200 of SEQ ID NO:3). The amino acid sequence containing the AT domain spans from a starting point of between about positions 901 and 1200 of SEQ ID NO:4 (ORFB) to an ending point of between about positions 1325 and 1400 of SEQ ID NO:4. The amino acid sequence containing the ORFB-AT domain is represented herein as SEQ ID NO:24 (positions 901-1400 of SEQ ID NO:4). It is noted that the ORFB-AT domain contains an AT active site motif of GxS*xG (*acyl binding site S₁₁₄₀).

ORFB-ER

The fourth domain in OrfB is an ER domain, also referred to herein as ORFB-ER. This domain is contained within the nucleotide sequence spanning from a starting point of about position 4648 of SEQ ID NO:3 (OrfB) to an ending point of about position 6177 of SEQ ID NO:3. The nucleotide sequence containing the sequence encoding the ORFB-ER domain is represented herein as SEQ ID NO:25 (positions 4648-6177 of SEQ ID NO:3). The amino acid sequence containing the ER domain spans from a starting point of about position 1550 of SEQ ID NO:4 (ORFB) to an ending point of about position 2059 of SEQ ID NO:4. The amino acid sequence containing the ORFB-ER domain is represented herein as SEQ ID NO:26 (positions 1550-2059 of SEQ ID NO:4).

Open Reading Frame C (OrfC):

The complete nucleotide sequence for OrfC is represented herein as SEQ ID NO:5. OrfC is a 4509 nucleotide sequence (not including the stop codon) which encodes a 1503 amino acid sequence, represented herein as SEQ ID NO:6. Within OrfC are three domains:

(a) two FabA-like β-hydroxy acyl-ACP dehydrase (DH) domains;

(b) one enoyl ACP-reductase (ER) domain.

The domains contained within ORFC have been determined based on: (1) results of an analysis with Pfam program, described above; and/or (2) homology comparison to bacterial PUFA-PKS systems (e.g., Shewanella) using a BLAST 2.0 Basic BLAST homology search, also described above. Sequences provided for individual domains are believed to contain the full length of the sequence encoding a functional domain, and may contain additional flanking sequence within the Orf.

ORFC-DH1

The first domain in OrfC is a DH domain, also referred to herein as ORFC-DH1. This is one of two DH domains in OrfC, and therefore is designated DH1. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1 and 778 of SEQ ID NO:5 (OrfC) to an ending point of between about positions 1233 and 1350 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-DH1 domain is represented herein as SEQ ID NO:27 (positions 1-1350 of SEQ ID NO:5). The amino acid sequence containing the DH1 domain spans from a starting point of between about positions 1 and 260 of SEQ ID NO:6 (ORFC) to an ending point of between about positions 411 and 450 of SEQ ID NO:6. The amino acid sequence containing the ORFC-DH1 domain is represented herein as SEQ ID NO:28 (positions 1-450 of SEQ ID NO:6).

ORFC-DH2

The second domain in OrfC is a DH domain, also referred to herein as ORFC-DH2. This is the second of two DH domains in OrfC, and therefore is designated DH2. This domain is contained within the nucleotide sequence spanning from a starting point of between about positions 1351 and 2437 of SEQ ID NO:5 (OrfC) to an ending point of between about positions 2607 and 2850 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-DH2 domain is represented herein as SEQ ID NO:29 (positions 1351-2850 of SEQ ID NO:5). The amino acid sequence containing the DH2 domain spans from a starting point of between about positions 451 and 813 of SEQ ID NO:6 (ORFC) to an ending point of between about positions 869 and 950 of SEQ ID NO:6. The amino acid sequence containing the ORFC-DH2 domain is represented herein as SEQ ID NO:30 (positions 451-950 of SEQ ID NO:6).

ORFC-ER

The third domain in OrfC is an ER domain, also referred to herein as ORFC-ER. This domain is contained within the nucleotide sequence spanning from a starting point of about position 2998 of SEQ ID NO:5 (OrfC) to an ending point of about position 4509 of SEQ ID NO:5. The nucleotide sequence containing the sequence encoding the ORFC-ER domain is represented herein as SEQ ID NO:31 (positions 2998-4509 of SEQ ID NO:5). The amino acid sequence containing the ER domain spans from a starting point of about position 1000 of SEQ ID NO:6 (ORFC) to an ending point of about position 1502 of SEQ ID NO:6. The amino acid sequence containing the ORFC-ER domain is represented herein as SEQ ID NO:32 (positions 1000-1502 of SEQ ID NO:6).

Example 2

The following example describes the use of the screening process of the present invention to identify three other non-bacterial organisms comprising a PUFA PKS system according to the present invention.

Thraustochytrium sp. 23B (ATCC 20892) was cultured according to the screening method described in U.S. Provisional Application Ser. No. 60/298,796 and as described in detail herein.

The biorational screen (using shake flask cultures) developed for detecting microorganisms containing PUFA producing PKS systems is as follows:

Two mL of a culture of the strain/microorganism to be tested is placed in 250 mL baffled shake flask with 50 mL culture media (aerobic treatment) and another 2 mL of culture of the same strain is placed in a 250 mL non-baffled shake flask with 200 mL culture medium (anoxic treatment). Both flasks are placed on a shaker table at 200 rpm. After 48-72 hr of culture time, the cultures are harvested by centrifugation and the cells analyzed for fatty acid methyl esters via gas chromatography to determine the following data for each culture: (1) fatty acid profile; (2) PUFA content; (3) fat content (estimated as amount total fatty acids (TFA)).

These data are then analyzed asking the following five questions:

Selection Criteria Low O₂/Anoxic Flask vs. Aerobic Flask (Yes/No)

(1) Did the DHA (or other PUFA content) (as % FAME) stay about the same or preferably increase in the low oxygen culture compared to the aerobic culture?

(2) Is C14:0+C16:0+C16:1 greater than about 40% TFA in the anoxic culture?

(3) Is there very little (>1% as FAME) or no precursors (C18:3n−3+C18:2n−6+C18:3n−6) to the conventional oxygen dependent elongase/desaturase pathway in the anoxic culture?

(4) Did fat content (as amount total fatty acids/cell dry weight) increase in the low oxygen culture compared to the aerobic culture?

(5) Did DHA (or other PUFA content) increase as % cell dry weight in the low oxygen culture compared to the aerobic culture?

If first three questions are answered yes, there is a good indication that the strain contains a PKS genetic system for making long chain PUFAs. The more questions that are answered yes (preferably the first three questions must be answered yes), the stronger the indication that the strain contains such a PKS genetic system. If all five questions are answered yes, then there is a very strong indication that the strain contains a PKS genetic system for making long chain PUFAs.

Following the method outlined above, a frozen vial of Thraustochytrium sp. 23B (ATCC 20892) was used to inoculate a 250 mL shake flask containing 50 mL of RCA medium. The culture was shaken on a shaker table (200 rpm) for 72 hr at 25° C. RCA medium contains the following:

RCA Medium Deionized water 1000 mL Reef Crystals ® sea salts 40 g/L Glucose 20 g/L Monosodium glutamate (MSG) 20 g/L Yeast extract 1 g/L PII metals* 5 mL/L Vitamin mix* 1 mL/L pH 7.0 *PII metal mix and vitamin mix are same as those outlined in U.S. Pat. No. 5,130,742, incorporated herein by reference in its entirety.

25 mL of the 72 hr old culture was then used to inoculate another 250 mL shake flask containing 50 mL of low nitrogen RCA medium (10 g/L MSG instead of 20 g/L) and the other 25 mL of culture was used to inoculate a 250 mL shake flask containing 175 mL of low-nitrogen RCA medium. The two flasks were then placed on a shaker table (200 rpm) for 72 hr at 25° C. The cells were then harvested via centrifugation and dried by lyophilization. The dried cells were analyzed for fat content and fatty acid profile and content using standard gas chromatograph procedures (such as those outlined in U.S. Pat. No. 5,130,742).

The screening results for Thraustochytrium 23B were as follows:

Did DHA as % FAME increase? Yes (38 −> 44%) C14:0 + C16:0 + C16:1 greater than Yes (44%) about 40% TFA? No C18:3(n − 3) or C18:3(n − 6)? Yes (0%) Did fat content increase? Yes (2-fold increase) Did DHA (or other HUFA content increase)? Yes (2.3-fold increase)

The results, especially the significant increase in DHA content (as % FAME) under low oxygen conditions, conditions, strongly indicates the presence of a PUFA producing PKS system in this strain of Thraustochytrium.

In order to provide additional data confirming the presence of a PUFA PKS system, southern blot of Thraustochytrium 23B was conducted using PKS probes from Schizochytrium strain 20888, a strain which has already been determined to contain a PUFA producing PKS system (i.e., SEQ ID Nos:1-32 described above). Fragments of Thraustochytrium 23B genomic DNA which are homologous to hybridization probes from PKS PUFA synthesis genes were detected using the Southern blot technique. Thraustochytrium 23B genomic DNA was digested with either ClaI or KpnI restriction endonucleases, separated by agarose gel electrophoresis (0.7% agarose, in standard Tris-Acetate-EDTA buffer), and blotted to a Schleicher & Schuell Nytran Supercharge membrane by capillary transfer. Two digoxigenin labeled hybridization probes were used—one specific for the Enoyl Reductase (ER) region of Schizochytrium PKS Orf B (nucleotides 5012-5511 of Orf B; SEQ ID NO:3), and the other specific for a conserved region at the beginning of Schizochytrium PKS Orf C (nucleotides 76-549 of OrfC; SEQ ID NO:5).

The OrfB-ER probe detected an approximately 13 kb ClaI fragment and an approximately 3.6 kb KpnI fragment in the Thraustochytrium 23B genomic DNA. The OrfC probe detected an approximately 7.5 kb ClaI fragment and an approximately 4.6 kb KpnI fragment in the Thraustochytrium 23B genomic DNA.

Finally, a recombinant genomic library, consisting of DNA fragments from Thraustochytrium 23B genomic DNA inserted into vector lambda FIX II (Stratagene), was screened using digoxigenin labeled probes corresponding to the following segments of Schizochytrium 20888 PUFA-PKS genes: nucleotides 7385-7879 of Orf A (SEQ ID NO:1), nucleotides 5012-5511 of Orf B (SEQ ID NO:3), and nucleotides 76-549 of Orf C (SEQ ID NO:5). Each of these probes detected positive plaques from the Thraustochytrium 23B library, indicating extensive homology between the Schizochytrium PUFA-PKS genes and the genes of Thraustochytrium 23B.

In summary, these results demonstrate that Thraustochytrium 23B genomic DNA contains sequences that are homologous to PKS genes from Schizochytrium 20888.

This Thraustochytrid microorganism is encompassed herein as an additional sources of these genes for use in the embodiments above.

Thraustochytrium 23B (ATCC 20892) is significantly different from Schizochytrium sp. (ATCC 20888) in its fatty acid profile. Thraustochytrium 23B can have DHA:DPA(n−6) ratios as high as 14:1 compared to only 2-3:1 in Schizochytrium (ATCC 20888). Thraustochytrium 23B can also have higher levels of C20:5(n−3). Analysis of the domains in the PUFA PKS system of Thraustochytrium 23B in comparison to the known Schizochytrium PUFA PKS system should provide us with key information on how to modify these domains to influence the ratio and types of PUFA produced using these systems.

The screening method described above has been utilized the identify other potential candidate strains containing a PUFA PKS system. Two additional strains that have been identified by the present inventors to have PUFA PKS systems are Schizochytrium limacium (SR21) Honda & Yokochi (IFO32693) and Ulkenia (BP-5601). Both were screened as above but in N2 media (glucose: 60 g/L; KH₂PO₄: 4.0 g/l; yeast extract: 1.0 g/L; corn steep liquor: 1 mL/L; NH₄NO₃: 1.0 g/L; artificial sea salts (Reef Crystals): 20 g/L; all above concentrations mixed in deionized water). For both the Schizochytrium and Ulkenia strains, the answers to the first three screen questions discussed above for Thraustochytrium 23B was yes (Schizochytrium—DHA % FAME 32->41% aerobic vs anoxic, 58% 14:0/16:0/16:1, 0% precursors) and (Ulkenia—DHA % FAME 28->44% aerobic vs anoxic, 63% 14:0/16:0/16:1, 0% precursors), indicating that these strains are good candidates for containing a PUFA PKS system. Negative answers were obtained for the final two questions for each strain: fat decreased from 61% dry wt to 22% dry weight, and DHA from 21-9% dry weight in S. limacium and fat decreased from 59 to 21% dry weight in Ulkenia and DHA from 16% to 9% dry weight. These Thraustochytrid microorganisms are also claimed herein as additional sources of the genes for use in the embodiments above.

Example 3

The following example demonstrates that DHA and DPA synthesis in Schizochytrium does not involve membrane-bound desaturases or fatty acid elongation enzymes like those described for other eukaryotes (Parker-Barnes et al., 2000, supra; Shanklin et al., 1998, supra).

Schizochytrium accumulates large quantities of triacylglycerols rich in DHA and docosapentaenoic acid (DPA; 22:5ω6); e.g., 30% DHA+DPA by dry weight. In eukaryotes that synthesize 20- and 22-carbon PUFAs by an elongation/desaturation pathway, the pools of 18-, 20- and 22-carbon intermediates are relatively large so that in vivo labeling experiments using [¹⁴C]-acetate reveal clear precursor-product kinetics for the predicted intermediates. Furthermore, radiolabeled intermediates provided exogenously to such organisms are converted to the final PUFA products.

[1-¹⁴C]acetate was supplied to a 2-day-old culture as a single pulse at zero time. Samples of cells were then harvested by centrifugation and the lipids were extracted. In addition, [1-¹⁴C]acetate uptake by the cells was estimated by measuring the radioactivity of the sample before and after centrifugation. Fatty acid methyl esters derived from the total cell lipids were separated by AgNO₃-TLC (solvent, hexane:diethyl ether:acetic acid, 70:30:2 by volume). The identity of the fatty acid bands was verified by gas chromatography, and the radioactivity in them was measured by scintillation counting. Results showed that [1-¹⁴C]-acetate was rapidly taken up by Schizochytrium cells and incorporated into fatty acids, but at the shortest labeling time (1 min) DHA contained 31% of the label recovered in fatty acids and this percentage remained essentially unchanged during the 10-15 min of [¹⁴C]-acetate incorporation and the subsequent 24 hours of culture growth (data not shown). Similarly, DPA represented 10% of the label throughout the experiment. There is no evidence for a precursor-product relationship between 16- or 18-carbon fatty acids and the 22-carbon polyunsaturated fatty acids. These results are consistent with rapid synthesis of DHA from [¹⁴C]-acetate involving very small (possibly enzyme-bound) pools of intermediates.

Next, cells were disrupted in 100 mM phosphate buffer (pH 7.2), containing 2 mM DTT, 2 mM EDTA, and 10% glycerol, by vortexing with glass beads. The cell-free homogenate was centrifuged at 100,000 g for 1 hour. Equivalent aliquots of total homogenate, pellet (H—S pellet), and supernatant (H—S super) fractions were incubated in homogenization buffer supplemented with 20 μM acetyl-CoA, 100 μM [1-¹⁴C]malonyl-CoA (0.9 Gbq/mol), 2 mM NADH, and 2 mM NADPH for 60 min at 25° C. Assays were extracted and fatty acid methyl esters were prepared and separated as described above before detection of radioactivity with an Instantimager (Packard Instruments, Meriden, Conn.). Results showed that a cell-free homogenate derived from Schizochytrium cultures incorporated [1-¹⁴C]-malonyl-CoA into DHA, DPA, and saturated fatty acids (data not shown). The same biosynthetic activities were retained by a 10,000×g supernatant fraction but were not present in the membrane pellet. These data contrast with those obtained during assays of the bacterial enzymes (see Metz et al., 2001, supra) and may indicate use of a different (soluble) acyl acceptor molecule. Thus, DHA and DPA synthesis in Schizochytrium does not involve membrane-bound desaturases or fatty acid elongation enzymes like those described for other eukaryotes.

While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims. 

1. An isolated nucleic acid molecule comprising a nucleic acid sequence encoding an amino acid sequence that is at least 95% identical to SEQ ID NO:24 or that is an enzymatically active fragment of SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 2. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 95% identical to SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 3. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is an enzymatically active fragment of SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 4. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 95% identical to SEQ ID NO:24, wherein said amino acid sequence acyl transferase activity.
 5. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding an amino acid sequence that is at least 95% identical to SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 6. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 96% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 7. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding an amino acid sequence that is at least 96% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 8. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 97% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 9. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding an amino acid sequence that is at least 97% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 10. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 98% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 11. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding an amino acid sequence that is at least 98% identical to SEQ ID NO:24 wherein said amino acid sequence has acyl transferase activity.
 12. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding an amino acid sequence that is at least 99% identical to SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 13. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding an amino acid sequence that is at least 99% identical to SEQ ID NO:24, wherein said amino acid sequence has acyl transferase activity.
 14. The isolated nucleic acid molecule of claim 1, comprising a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:24.
 15. The isolated nucleic acid molecule of claim 1, consisting of a nucleic acid sequence encoding the amino acid sequence of SEQ ID NO:24.
 16. A recombinant nucleic acid molecule comprising the nucleic acid molecule of claim 1 and a transcription control sequence.
 17. A recombinant plant cell that expresses the nucleic acid molecule of claim
 1. 18. A recombinant microbial cell that expresses a recombinant vector comprising the nucleic acid molecule of claim 1 and a transcription control sequence.
 19. The recombinant microbial cell of claim 18, wherein the microbial cell is a bacterium.
 20. The recombinant microbial cell of claim 18, wherein the microbial cell is a Thraustochytriales microorganism.
 21. The recombinant microbial cell of claim 20, wherein the Thraustochytriales microorganism is a Schizochytrium or a Thraustochytrium.
 22. A method to produce at least one polyunsaturated fatty acid (PUFA), comprising culturing under conditions effective to produce the PUFA, a plant cell or a microbial cell that expresses a PKS system for production of PUFAs, wherein the plant cell or microbial cell expresses a recombinant vector comprising the nucleic acid molecule of claim
 1. 